I think this is a wrong approach. If you'd really like to give them access to raw mail files, simpy back up /var/qmail/mailnames and send the archive to your new host. Or, better, if you'd like to transfer mailbox contents in an organized manner, use IMAP for transfer.
For automated IMAP transfers, I recommend imapsync. The program is available in EPEL for CentOS and RedHat, while Ubuntu and Debian users need to install it manually.