IDEA: Corpses possessed and animated by disembodied fiends. They could count as both undead and evil outsiders. Fiend-associated necromancers could make them/propagate them. It could be a new monster.
OR
We could simply add the fiendish template to any undead and give it the mental ability scores of he fiend possessing it.
Just a thought.
I don't believe possession rules make the host count as the possessor's type. They would remain undead, just controlled by outsiders and maybe with some extra powers.
See Shadow Demon.