The facility is there ...
If you get the voice definition file, you can add it to the ~/Library/Voices folder.
That said, I haven't seen any extra voices floating around, but then I've never really looked for them.
I'm pretty sure that the voice description files are not simple text files like .plist files are.
I've been Googling and Yahoo!ing since this thread was created, and have not found any information on adding voices to the Mac system, either for the classic OS or for OS X.
I think it would be safe to assume that no other voices exist for Mac OS X at this point in time.