All the voices belong to the Speech synthesizer and will say whatever you tell them to. They are not stored as individual sound files.
With that in mind, you could take an audio recording program, Audion for example, and set it to record.
Before doing that, open TextEdit and type in whatever you want the voice to say.
Start recording and then start the voices speaking.
Once finished, edit your sound file and save it.

Any good?

P