I think it can be changed using LETO, so you'd have to get someone to do it for you.
I actually used an NPC voice set when making Xiao, and it worked great, until there was an update to NWN. Since then, the voice is still the same, but the text that goes along with it is from a different voice set! Usually the comments are close enough to not make much of a difference, but there are some rather hilarious ones, too. It's best seen in-game, of course. I never botherd to change it because it's good for a laugh from time to time.
