this is a difficult question even I am not sure of answering, but here might be a sugestion.
# you could try to make a mercenary and just make a custom unit (Via crtl+shift+n), you change the model of the unit to a death corpse (look at the model of the unit), and then you should add the unit as purchable at the mercenary you have made, when you buy it you see a death corpse and maybe then the necromancer could turn it into a skeleton.
If this doesn't work witch will i think occur than, you should just play with the editor a long time till you figured out what to do.
Good luck
/Warnicro/