The mb2md script is a simple way to convert any mbox files over to Maildir.
wget http://batleth.sapienti-sat.org/projects/mb2md/mb2md-3.20.pl.gz
gunzip mb2md-3.20.pl.gz
mv mb2md-3.20.pl mb2md
chmod 755 mb2md
Once setup, you can then convert the account.
Here is a sample for converting virtual pop acounts
./mb2md -s /var/spool/virtual/domain.com/user -R -d /home/username/imap/domain.com/user/Maildir
cd /home/username/imap/domain.com/user/Maildir/cur
chown username:mail *
chmod 660 *
To convert the main system account:
./mb2md -s /var/spool/mail/username -R -d /home/username/Maildir
cd /home/username/Maildir/cur
chown username:mail *
chmod 660 *
The same method can be used to convert the mbox imap folders as well, eg: