Audition does not currently offer any tools for replacing voices in this manner. The OS X command line enables you to perform a text-to-speech conversion from a text file source and output to an audio file in one of a number of voices.
say -f textfile.txt -o outputfile.aiff -v voicename
The "voicename" can be determined easily by opening the System Preferences dialog and previewing the Speech settings.
I am unfamiliar with other realistic text-to-speech applications on Windows or Mac, so perhaps other forum users can make suggestions.