I'd probably treat it like a custom Monstrous Career (Eons 141) and bump the Maxdie pool by 1, making the natural attacks (such as any bites, scratches, etc) use that diepool instead, and give it a ccouple undead traits like:
- darksight
- a +1d6 bonus of necromantic damage (Or maybe turn the base damage into Necromatic damage and add a special Zombie Disease trait like the Plauge rat has as an alternative)
- 2d6 vulnerability to Holy or Radiant damage (depending on campaign stye of future or magic) and an extra 4 SOAK to Peircing and blunt damage.
Or whatever similar flavor traits like that you want.