mancerbear said:
It would certainly be highly unusual, but you are of coarse able to to whatever you wish in YTU. I'd be a bit worried it would break the genre for the players unless you could come up with a vslid psuodo-science explaination for it. Such as the undead are the result of an alien fungus infection.
This would be my intention. Magic effects could be simulated by psionics or highly advanced science like the technomages in Babylon 5 (or is there already anything similar to technomages?). Supernatural effects like zombies or dragon breaths with infections and chemistry. A conversion guide could come in very handy or maybe an expanded list of traits based on the animal creation in the core rulebook. Coming from D&D like games and being new to Traveller, some guidelines really would be very helpful.