I would suggest that a noble should't be a necromancer. However, I think that there should be a small chance that a dwarf having long periods of exposure to corpses and miasma could become some kind of degenerate, and if he is then struck by a fay mood he could become a necromancer and start building some kind of frankendwarf or raising zombies or somesuch. Maybe he performs a curse that causes him to come back as a vampire dwarf after he dies... Lots of possibilities.
You could "force" Necromancer dwarves to be created by locking them in a room specially prepared with rotting corpses (dwarf corpses should only count). But to prevent exploitation, maybe only 1 in 8 dwarves would actually become necromancers (and they'd still need a fay mood to do it). The rest would just go insane or become severely depressed when exposed to that much death. This way there's a bit of a risk involved.
Once you've got your necromancer(s), his skill can improve as he experiments with his abilities. Maybe even have a special workshop that only gets unlocked after you have your first necromancer. There he can make more powerful versions of undead without having to use the other workshops. I would imagine frankendwarves would be made at craftsdwarf shops during fay moods...
One interesting thing could be allowing a vampire dwarf to eventually become a noble or to attract a vampire noble. Maybe there's a dark order of vampire dwarves hidden away somewhere with their own nobles who only get involved in fortresses where necromancy is accepted. Could even base some interesting subplots on this...
For those who don't want to accept necromancy, there could be an option on the justice window that toggles "Necromancy is illegal". Then any dwarf found to be practicing necromancy gets jailed/executed.
[ December 19, 2007: Message edited by: Rob Allen ]