So its a really long story and in the end irrelevant as to the how and why this scenario came about but suffice to say I just ran my players though a short scenario where they made a short visit to a Space Hulk. The particulars are relatively irrelevant other than to say they both they and this particular Hulk were in transit to a Hive World and that the Hive world had been brought into utter anarchy by a Nurgle oriented Chaos Uprising. Like I said its a ridiculous and stupid story as to the how and why a Hulk got involved but I had to find a way to make the two connected. So the end result was the Hulk was called "Pandora's Dirge". They get a short briefing on some of the incidence that this particular Hulk has been a part of over the millenia. Every instance points very vaguely and lightly as a reference to one of the Chaos Gods (mostly just for fun) but none of the players picked up on it. No biggie. Anyhow; it was Papa Nurgle's turn to have a go with the Hulk and he was influencing things to guide the Hulk to this Hive world. I wanted to go with something unexpected and creative that my players (most of whom are very familiar with the fluff) would not expect. I was a fan of the Original Space Hulk game and so that got me thinking about a Genestealer Coven developing on the Hulk. Then I though; well what would happen if a Genestealer started it's coven by grabbing up a Human who was a Chaos Cultist. In this case a Cultist dedicated to Nurgle. Well maybe that would end up with Genestealer Hybrids that also worshiped Nurgle. So I downloaded the Genestealer Cult fan product from Dark Reign and looked up some of the nasty disease bits from the Tome of Decay and there I was with a nice blend of Genestealers, Hybrids, and nasty virulent disease. The Disease made the Baddies more dangerous in its own right but also seriously detracted from some of their typical key strong points. When my players ran into the signs of Genestealers they were rightfully concerned. When they discovered that there was Nurgle taint involved they **** themselves. End result was it ended great and was a lot of fun. I did have one player though that seemed to take serious issue with that scenario as being a break from Cannon. We had they discussion and my side of it boiled down to forces in the galaxy aren't simply against Humanity, they are against one another as well. If Chaos can corrupt Primarchs then I cannot see a reason why it would not be able to corrupt Xenos as well. He didn't seem convinced. I was curious if you fine gentlemen had any similar instances?

Chaos corrupts just about everything, so with Tyranids being just yet another "normal" lifeform, they can be twisted and mutated into something new just like animals and plants. It's certainly something that would not happen very often as there are a number of mitigating factors, though.

Also, this corruption would probably require physical exposure for directly "infecting" the Tyranid creature/s, rather than psychic weakening as it works on human psykers: even though the Tyranids are inherently psychic as well, the hivemind's presence is so vast that it'd probably drown out any psychic attack a single daemon could muster. Plus, the 'nids are instinct-driven creatures, so whilst Chaos manipulation could probably divert or distract a swarm, it would never be subservient.

With Genestealers, on the other hand, things might be a little different, as they're not yet part of the Hive Mind and the 'stealer Patriarch is capable of independent thought (within the perimeters of their instinctive "programming").

What I'd change about your story is to either make sure the 'stealers are exposed to direct corruption by a Chaos artifact, or the Chaos cultist already being infected with a gift from Papa Nurgle. The Genestealers would certainly not start liking Chaos just because one of their victims did.

Anyways, here are two incidents from the 3E codex Daemon Hunters, from the list of "story hooks" back when GW still bothered to provide examples for reasons to fight an enemy:

• "A hive ship was sucked into the Warp and what emerged was corrupted beyond all reason. When the few surviving monsters make planetfall, the Daemon Hunters are waiting to destroy them all."
  
• "A corrupted Hive Tyrant has been infected with Nurgle's Rot and is spreading plague ahead of its swarms. The Daemon Hunters must find and kill it to give the Imperial forces a chance at resisting."

Or that one where an imperial ship activates its warp drive the moment it is about to to ram a tyranid hiveship and sends it straight into the warp in the middle of a congregation of bloodthirsters wich promptly tear the 'nid ship apart.

Screw corruption. Aquire skulls for the skull throne instead.

Anybody remember back in the day when genestealer cults had Khorne worshipers?

Well that was, I think, back before Genestealers were really associated really strongly with the rest of the Tyranids.

Still awesome/wacky old fluff. I really want to run or play in a DH game where the genestealer cult is the main antagonist.

Does anyone know what the 'mindstealer sword' as described in that is?

From what I got from skimming trough the Rogue trader Compilation and Realm of chaos it's a chaos* weapon with the mind eater property. In that case it would deal no phisical damage (wounds) when it hits, but instead reduces the target's INT, WP ( and Ld and Cool) to zero. the target becomes subject to stupidity and looses all skills. (These are the rules for 1st edition WHFRP/ WHFB/ WHK40K so you might wanna change the rules a bit.)

* The book explains the whole 'stealer chaos thing. This is what happens when a chaos cult gets infected by a purestrain genestealer, and you end up with a 'stealer cult who have acces to mutants, spawn, chaos weapons/gifts, summoned daemons and... horses for some reason.

You should be able to placate your angry player by pointing out that, as a Nurgle cult, the people infected by the genestealers had superhuman resilience; thus instead of their human DNA being rendered subservient to 'stealer DNA, their superhuman constitution made the 'stealer DNA subservient to their own. *handwave complete*