The custom greetings part is easy.  I used 'sox' to convert the old greetings I had in 'au' files to /usr/local/freeswitch/storage/voicemail/default/domain/box/greeting_1.wav  I then manually selected greeting 1 from all the voicemail accounts, but I found later that a simple sqlite3 script on the /usr/local/freeswitch/db/voicemail_default.db database can do that.  Importing the old messages can also be done with a script, but it was too much trouble to be worth it for me.