I made a quick conversion for the Dark Eldar Race and the Archetyp Kabalite Warrior from Rogue Trader: Soul Reaver. I would be glad about comments, criticism and help. Sorry for any spelling mistakes, I am not as fluent in english as I like to be.

Races:

Dark Eldar
Dark Eldar generates there starting Characteristics as Human character and begin with 500 start Experience Points. They generate Gift of the Gods like Chaos Space Marines, but instead of Gift of the Gods the drain on their souls strengthen. The Dark Eldar character needs an additional Pain Token to generate an temporary Infamie Point for every Gift of the Gods and they start to lose characteristic a day sooner when not generating a Pain Token in that time frame.
Dark Eldar Starting Abilities
All Dark Eldar characters begin play with the following Skills, Talents, Traits, and Equipment.
Characteristic Bonus: +5 Agility, +5 Perception
Starting Skills: Common Lore (Dark Eldar), Intimidate, Linguistc (Dark Eldar)
Starting Talents: Catfall, Jaded, Heightened Senses (Sight, Hearing), Paranoia, Sprint
Starting Traits: Dark Sight, Power Through Pain, Unnatural Agility (+3), Vile Pleasures

Achetypes:

Kabalite Warrior
Skills, Talents & Gear
Kabalite Warriors must be Dark Eldar.
All Kabalite Warriors begin the play with the following:
Characteristic Bonus: +3 Weapon Skill and +3 Ballistic Skill
Starting Skills: Acrobatics, Awareness, Common Lore (War or Screaming Vortex or Koronus Expanse), Dodge, Linguistic (Dark Eldar) +10, Linguistic (Eldar), Medicae or Deceive or Trade (Chymist), Parry, Stealth
Starting Talents: Disturbing Voice or Leap Up or Pity the Weak, Hard Target or Betrayer or Combat Master, Kabalite Weapon Training, Resistance (Posion), Weapon Training (Primary, Chain, Power, Shock)
Starting Gear: Best Craftmanship splinter pistole or Good Craftmanship splinter rifle or Common Craftmanship shardcarbine, Best Craftmanship mono-sword or Common Craftmanship power sword, Kabalite armour, micro-bead, tramsöator unit, gruesome trophy or kill-records, 2 doses of any one poison or 3 doses of any one combat drug
Starting Wounds: 10+1d5
Special Ability:
Cold Killer
See Special Ability of the Chaos Space Marine Chosen.
Kabalite Warriors begin play aligned to Slaanesh and can never change their alignment.

Talents

Agony From Afar
Alignment: Tzeentch
Tier: 2
Prerequisites and functions exactly like in the book Rogue Trader: Soul Reaver.

Cruelty
Alignment: Khorne
Tier: 2
Prerequisites and functions exactly like in the book Rogue Trader: Soul Reaver.

Disciple of Shiamesh
Alignment: Nurgle
Tier: 3
Prerequisites: Scholastic Lore (Chymistry) +20, Trade (Chymist) +20
This character may spend a Infamie Point to automatically pass a Trade (Chymist) Test, succeeding by a number of Degrees of Success equal to his Intelligence Bonus. In addition, when wielding any weapon which has the Toxic Quality, the Dark Eldar adds his Intelligence Bonus to the weapon's Damage. He also adds the Felling (1) Quality to the weapon if it does not already have that Quality, or increases its Felling Quality by 1 if it does.

Kabalite Weapon Training
Alignment: Unaligned
Tier: 1
Prerequisites and functions exactly like in the book Rogue Trader: Soul Reaver, except not covering Primary Weapons.

Siphon Pain
Alignment: Slaanesh
Tier: 2
Prerequisites and functions exactly like in the book Rogue Trader: Soul Reaver.

Surgical Precision
Alignment: Slaanesh
Tier: 2
Prerequisites and functions exactly like in the book Rogue Trader: Soul Reaver.

Tormentor's Fury
Alignment: Khorne
Tier: 2
Prerequisites: Dark Eldar, Power Through Pain
After gaining at least one Pain Token in an encounter, the character may spend a Infamie Point to gain the Berserker Charge Talent and the Brutal Charge Trait (2) for the remainder of the encounter.

Tormentor's Majesty
Alignment: Slaanesh
Tier: 3
Prerequisites: Dark Eldar, Power Through Pain
After gaining at least three Pain Tokens in an encounter, the character may spend a Infamie Point to gain the Fear (1) Trait for the rest of the encounter. While he benefits from this Fear Trait, he also adds +10 bonus to all Intimidate and Command Tests.

Tormentor's Might
Alignment: Khorne
Tier: 3
Prerequisites: Dark Eldar, Power Through Pain
After gaining at least three Pain Tokens in an encounter, the character may spend Infamie Points to gain the Unnatural Strength (x) Trait. There by x equals the number of used Infamie Points till a maximum of four.

Tormentor's Supremacy
Alignment: Nurgle
Tier: 3
Prerequisites: Power Through Pain, Tormentor's Vigour
After gaining at least four Pain Tokens in an encounter, the character may spend two Infamie Points to gain the Stuff of Nightmares Trait for the remainder of the encounter.

Tormentor's Vigour
Alignment: Nurgle
Tier: 3
Prerequisites: Power Through Pain, True Grit
After spending a Infamie Point to heal himself, the character can spend additional Infamie Points to gain the Regeneration (x) Trait for the remainder of the encounter. There by x equals the number of used Infamie Points till a maximum of five. The Trait only functions in rounds in which the character received at least one Pain Token.

Tormentor's Will
Alignment: Slaanesh
Tier: 3
Prerequisites: Power Through Pain
After gaining at least two Pain Tokens in an encounter, the character may spend a Infamie Point to gain the From Beyond Trait for the remainder of the encounter.

Traits

Power Through Pain
This Trait functions ecaxtly like in the book Rogue Trader: Soul Reaver, but Dark Eldar Characters who actively seek out alliances with the disciples of chaos are dangerously close to Slaanesh and losing there soul. Every they confront the presence of chaos head on and feel the drain of there souls and spirit more heavily. Therefore they need twelve Pain Tokes to gain a temporary Infamie Point. They also lose 1d5 from every characteristic every week if they do not generate at least one Pain Token. If there generate a Pain Token there characteristic recovers instantly.

Vile Pleasures
This Trait functions exactly like in the book Rogue Trader: Soul Reaver.

Personal point of view here...

I would say that a Dark Eldar would be unaligned but would count as allied for the purpose of purchasing skills and talents aligned with Slaanesh and Khorne. After all, they cant ever gain Gifts or Rewards so there must be some tangible benefit towards playing them. And many Dark Eldar revel in close combat, Khorne and Slaanesh holds those talents.

Or go to the other extreme and say that they are already deeply corrupted but has made a deal with She Who Thirsts for extended life and therefor only serves to satisfy ITs cruel thirsts. Every other Pain Token they earn goes directly to She Who Thirsts which means they need to earn 20 Pain Tokens to gain a temporary Fate/Infamy Point. But they can get Gifts and Rewards as normal. I still would like them to be "Allied" to Khornes Skills and Talents(not Characteristic, ie Strength), as well as Slaanesh's though.

In fact I would say.

Weapon Skill - Allied

Ballistic Skill - Allied

Agility - Allied

Strength - Opposed

Toughness - Opposed

Fellowship - Opposed, can get to Unaligned if Highborn, see talent etc

Rest of the characteristics Unaligned

Khorne and Slaanesh's Skills and Talents are allied, the other Powers skills and talents are Opposed.

Just a few thoughts... Been thinking about the same subject after all

Worked out something similar to this using Soul Reaver & Dark Kin

I figured track Corruption as normal, but instead of mutations/gifts, increase the Pain Tokens thing by +1 per landmark

Example: 11 tokens needed per at 10 corruption, 12 at 20, 13 at 40... etc (I used human corruption track)

Opted not to give them a 'class power' either - the pain token thing seemed like enough

Sorry totally forgot about this topic, but interesting ideas.

@Drachdhar : Well I think being aligned to two gods would be a rather great boon, because nobody else could get two cheap gods. And I found in Rogue Trader the Pain Tokes to be rather strong and easy to acquire fate points with the right choice of talents. Therefore I found them a rather great boon for the fact that they got no mutations or gift of the gods. But maybe one could try it out. But another idea for other careers would be other alignments. That would maybe separate the other career for Dark Eldar quite nice. Maybe I should think about it.

@DarkHammer : Well this I also did, but gave them class power. It seemed rather fitting, but I could not find out as today, if it is really unbalanced or not, because I could only test it in a play-by-post round, which is rather slow. In my role playing group nobody wanted to play one, except for a one-shot adventure.

I wouldn't see the Dark Eldar ever willingly taking part in Chaos ever. That simply isn't the way they are, as funny as it is to say that. Baby eating, child slaving, sexual deviancy, torture for the fun of it, unzipping you and wearing your skin as a coat, but never Chaos! To do so would be to have their very souls devoured, and that isn't really on the active agenda for any Dark Eldar.

Using alignment mechanics simply to dictate advancement costs I could see, however, this would most certainly deviate them in a significant manner from their Rogue Trader counterparts in the costs and availability of their advances, as certain advances may be much cheaper, or much more expensive than what a typical Dark Eldar would pay in RT, the game they are actually balanced for.

A question, if you have access to Soul Reaver, why not just use them in the same way as they are in there (ie, Career Class, non-variable XP costs, etc), and just port them directly? Why convert them? The rules are more or less compatible, and given that they are Xenos who do not actually worship the Dark Gods (as opposed to Black Crusade's typical Human Chaos Worshipers), it should be expected that they play by different rules to a degree.

If you really want to convert them to the BC advancement system however, one thing you could do is to use Soul Reaver as your base for XP costs, and for anything NOT covered by their Career Classes in RT (ie, Elite Advances in that system), align them to Slaanesh for the purposes of XP cost, because whether they want to admit it or not, they are Her true children, and connected to Her whether they want to be or not. XP costs in Soul Reaver are comparable to those in Black Crusade from what I can discern, and a Dark Eldar character would not be paying much more or less than a Black Crusade character for various skills and talents.

Of course, you could always bring in that one Dark Eldar character who is just a bit too weird for even his own kind, and say, is willing to make a deal with Khorne to avoid having his soul devoured by Slaanesh. He would no longer count as a "typical" Kabalite, but his deviancy could justify the different system mechanics from RT that you are attempting, and make for an entertaining character to play