Migrating from Prosody's flat files data storage
Basically this is a very simple and straight forward process, you just copy the Prosody's data store to the Metronome's datastore path (usually on a debianish global install that is /var/lib/prosody for Prosody and /var/lib/metronome for Metronome), perhaps:
sudo cp -r /var/lib/prosody /var/lib/metronome
sudo chown metronome.metronome /var/lib/metronome
On a linuxlike... obviously since the flat files format is the same and mostly compatible the reverse is possible as well.