Neela, thanks for the pointer - it could help if we want to do lots of distinctly-voiced NPCs and aren't good at putting on funny voices naturally.
Deathbane27, I don't think our own custom NPCs will be a problem... we can voice all the dialog for them, including all the standard stuff. Well, not a problem apart from lame voice acting, at least in my case

I was specifically concerned about adding an extra bit of dialog to an existing NPC, in order to tie your quest into the larger game. Like, adding some extra dialog to the arch-mage of Cryodil so he can send the player off to your pocket universe or something, while leaving his other conversational gambits intact.
So, to add a bit of extra conversation to an existing NPC, the options as I see them are:
* throw away Bethesda's voice actor's work and re-voice
all the target NPC's dialog (which would be a bit intrusive, and could be a lot of dialog for important NPCs)
* find a fan who's a good vocal mimic to record the new lines for you
* have the new lines spoken in a different voice (could work if the NPC was suddenly possessed or something, but that could only be used in moderation, not for every extra piece of dialog on existing NPCs)
* have the new lines un-voiced.
* find some software that can sample the existing dialog and allow you to synthesise new lines using the sampled voice, if such software exists.
Any other ideas?