It should be fairly trivial to write an extension that does this for the user but it would be outside the VoicemailMain app.
Just play back the greetings from the filesystem with Playback.
exten => 100,1,Playback(/var/spool/asterisk/voicemail/kiniston/7011/greet)
same => n,Playback(/var/spool/asterisk/voicemail/kiniston/7011/busy)
same => n,Playback(/var/spool/asterisk/voicemail/kiniston/7011/unavail)
You can always use variable substitution to build the path dynamically for your users.
Look at the CHANNEL function and SIPPEER functions to get that info.