As far as 'importing' goes.. when i migrated my archives, i just tarballed them on old server (/var/vmail folder specifically), ftpd them to new server, then did a unatr in root preserving timestamps, otherwise you end up with an inbox with thousands of new emails with todays date and time stamps. made the example.com mail server entries, opened roundcube to populate specific folders cos im lazy, and copied the stuff untar dir to the respective spots.. yeas its manual but literally takes less time to move than it does the tar from original server.