I'm posting this on the Rogue Trader forum, although the Xenos in question are originally from Dark Heresy's Creatures Anathema book. As such, this thread contains minor spoilers for the Creatures Anathema book.
I understand that the Enoulians seem to be invented purely for the role-playing games without anything similiar in the board games. As a minor threat and a footnote, these little oompa loompas don't get the love (or in WH40k context, the hate!) they might deserve. So, let's put this right!
First of all, there might be a hidden Enoulian plot hook in Rogue Trader regarding the Enoulians. Their weapons are Shard weapons (rifles, pistols) and in RT there's a Shard battery for starships. Enoulian design, perhaps?
Here are a few plot hooks I have used:
THE ENOULIAN REFUGE: a star hidden deep in the Koronus Expanse houses the last remains of the Enoulian civilization destroyed in the Angevin Crusade. The xenos still thrive, hidden on an agri-world like planet from the Emperor's Wrath. Ruled by three castes - the religious, the military and the worker caste - they bicker amongst themselves how to best reap vengeange against the Imperium. As such, a ruthless Rogue Trader may exploit their hatred to his own purposes.
THE ENOULIAN SOCIETY: The religious caste flirt secretly with chaos rituals, but are open to negotiations with the Rogue Trader, wishing to use them to their advantage. The worker caste are the engineers and labourers of the Enoulian Refuge. The worker caste manufactures the shard devices, being the main source of power on the Enoulian weapons, hives and ships. The military caste can be found around Calixis as mercenaries, but they hate the Imperium with a passion. Seeing an aquila sends them in a Frenzy, unless they succeed in a Willpower test. Also, with the worker caste, they are slowly building a fleet of Enoulian warships and trade weapons and ship parts with pirates, their ultimate goal to strike against the hated bearers of the Aquila en masse .
PROVING ONE'S WORTH TO THE ENOULIANS: Being a culture at war, disagreements in the Enoulian parliament are settled with martial melee combat. If the Rogue Trader wishes to negotiate with all the factions in the Refuge, they must send a champion to face theirs and their whisper lines in the Parliament arena, in front of hissing crowds of Enoulian caste leaders. Of course, a shrew negotiator might persuade the religious or worker caste not to challenge their right to trade with the Refuge. The warrior caste, of course, hates the Imperium and is convinced that if they leave, they will return with a war fleet and will gladly send a champion to spill human blood onto the arena - as well as warships to sneak up on the Rogue Trader's ship as they leave the Refuge.
TRADING WITH THE ENOULIANS: The Enoulians may trade shard weapons, shard power batteries, whisper lines and perhaps even fit the Rogue Trader's ship with Shard batteries if they succeed in bartering with them. As such, a smuggling route from the Refuge to, for example, Footfall could be a profitable greater endeavour. In return, they request odd trinkets: tons dirt from human agri-worlds, piles of bones from dead human children. Odd and sinister perhaps, but seemingly harmless.
THE GREAT DESIGN: But of course, the Enoulians have a plan. Using the know-how of the worker caste and the magicks of the religious caste, they plan to forge warp-touched xenotech bioweapons that cause infertility in both crops and Imperial colonists (using the dirt and bones to forge said weapons). Once the Enoulian warp virus bombs are ready, do they trust the enough Rogue Trader to ask her to smuggle their deadly weapons and their handlers into the Imperium or will they use their pirate allies or their war fleet to attack the human agri-worlds near the Expanse? In any case, a Rogue Trader trading with the oily-skinned aliens may soon hear disturbing news from the Imperial worlds... and as such, an Inquisitorial query might not be far off.
XENOS, XENOS, XENOS!: As the Enoulians will trade with anyone to achieve their goal of ultimate vengeange, one might add even more Xenos factions into the refuge. A Kroot mercenary colony working for the Enoulians? An unlikely Ork ally with their ship? A visiting Eldar overseer, worried about the Enoulian warp experiments? A faction of humans, escaped from the Angevin Crusade generations ago, now living amongst the Enoulians? The other factions might also have a vote - and a champion - in the Enoulian Parliament.
THE ENOULIAN SHIPS: Fregates, transports and raiders as generated by the GM's toolkit, but with Shard weapons instead of the usual macrobatteries. Also, the warrior caste uses swift attack ships, slippery as the their Enoulian pilots themselves, giving them a +10 piloting when attacking in Hit & Run. The warrior caste ships sport a Crack crew, while the other casters have ships with Regular crews.
ENOULIAN MAGICKS: The religious caste high priests have a (sorcerous) Psy Rating of 3, with relevant psychic powers from the biomancy discipline. They will attack with Blood Boil and Bio-Lightining if their true nature is revealed and they are put against the wall.