A quick way to convert Mbox mailboxes to Mbox format is to use mb2md.
Install it in Ubuntu/Debian using:
sudo aptitude install mb2md
To make the convertion use:
mb2md -mFrom man:
-m If this is used then the source will be the single mailbox at /var/spool/mail/blah for user blah and the destination mail‐
box will be the "destdir" mailbox itself.
Nice and simple!