I'm planning on running an "Aliens" themed game for my home group and decided to use Rogue Trader as the system, with some modifications. I wanted to create the xenomorph from scratch but I don't see any rules in the core book for threat creation. Did I miss it? Page 364 starts the chapter on Adversaries and Aliens but only lists various traits.

Any suggestions?

Kronus bestiary has a xenos creater in it if I remember correctly.

Awesome, thanks! I'll have to head over to the flgs this weekend.

Honestly i'd suggest using Only War for the player characters and Tyranid stats gathered from all of the 40k RP lines for the adversaries. Unless of course you want starship stats from Rogue Trader, but those could be imported.