I found a way to export your personal mailbox greetings that works well. You'll need the login info for each of your users to the MAS portal if running a NuPoint in a MAS, otherwise I'm not sure if te dedicated server version of the NuPoint has the 'portal' webpage.
Once logged in, select the mailbox from the menu on the left then select the Greetings tab. Checkmark the greeting you want to download and in Chrome or IE, clicking "Play" will cause the WAV file of the greeting you selected to be downloaded automatically (Chrome) or you'll get a prompt to do so (IE). Not sure how Firefox behaves.