The same way Genestealers get anywhere - someone was careless in what they brought onboard the ship. Here's how I would explain it:
It would be totally possible that members of a human genestealer cult would be unknowingly picked up in a slave raid. Their captors probably wouldn't notice at first, especially since Dark Eldar are pretty deadened psychically. The 'stealer cult wouldn't get much of a chance to spread through the slaves as the hybrids would be caught quickly, but let's say someone was picked up pregnant and ready to give birth to a purestrain. THAT would be trouble, as even the
DE would be hard pressed to fight off a genestealer sneaking around and dragging off random eldar who venture into the slave pits.
So now your slave pit guards are brood brothers. They would allow the cult to spread through the humans, until it was powerful enough to overwhelm the Wyches. Guards release and arm prisoners, take wyches by surprise and allow the purestrain(s) to forcibly implant them with the genestealer gene seed, et voila. You now have a Wych Cult whose
DE members are a first
gen part of the brood (so brood brothers, no mutations) accompanied by humans from all different generations, including hybrid and purestrain.
(even the impossible is possible if you bend the fluff enough)
Automatically Appended Next Post:
Tiarna Fuilteach wrote:A boarding of a dark eldar ship by tyranids who allow the wych to escape as she has been infected. I was more basing it on the fact that in commoragh even the most disturbing acts go unheeded by anyone
Two problems with this: one, dark eldar don't reproduce naturally (at least not often enough or fast enough to spread a genestealer cult), which leads to two, you need the purestrain 'stealer to infect more dark eldar.