Okay, the way I'd do it is to create three new files in your RAW\objects folder.
First one : body_undead.txt. Make sure the first and second lines are body_undead and [OBJECT:BODY] respectively. Then copy/paste the spoilered text under body in the mod's first post.
Second: creature_undead.txt. Again, fist two lines should be creature_undead and [OBJECT:CREATURE], then copy/paste again.
Third: Repeat the above, but with entity_undead.txt. Again, first two lines should be entity_undead and [OBJECT:ENTITY], and copy/paste.
That
should gen worlds with the undead griblies showing up. I'd recommend against simply adding the respective texts to the end of any vanilla files, try and keep your modded stuff in separate files.
Edit: Actually, if you want to be
really evil, you could remove all entities except for dorfs and the undead. Then we'd not have to worry about other civs. Of course, it may be fun letting them gen, and watching the slaughter as an elven caravan wanders into a sieging pack of zombies, skellingtons and ghouls..