interesting... unnamed dwarvish undead get coffins. I have genericcoffins with "dwarf corpse", "peasant right upper arm", "peasant partial skeleton", "peasant right lower arm", "peasant partial skeleton", and "peasant partial skeleton".
I've thus made a new corpse stockpile, and am going to dump all that crap before I fill up 20+ coffins with body parts of a couple unnamed non-citizen dwarves.
Yeesh, I had that too. They even entombed a few necromancers.
Lucky for me, I had killed a better part of the undead by retracting my pair of entrance drawbridges underneath them, dropping them into the volcano I had made my entrance, so there were fewer corpses to entomb. With a squad of marksdwarves who did a very impressive job of fending off them off and mopping up the stragglers, I won quite easily.
...except that the zombies I dropped into the volcano didn't die. They just sat there, at the bottom of the magma sea, and the "siege" status never went away.