I see only one problem with this and it's how is he going to have a voice outside of his soundfiles in the Mojave Wasteland. I guess they could figure out a way through modding but I'm not sure.
If the modder in question has audio editing equipment it's actually not that hard to take cuts from the current sound files and splice them back together to make additional speech. Time consuming? Hell yes, but not particularly difficult. Generating the .lip files for the facial movements might be a pain, but I don't do much of that so I could be wrong.
Making the appropriate dialogue trees with all their conditions would take a while, as would making any companion quest. The various animations are probably already there so those would just have to be C&P over to the new character.
I'm not about to try this myself as I svck at making NPCs, but this should at least give you an idea of what someone who's good with them has to do to make one.