A quick way to convert Mbox mailboxes to Mbox format is to use mb2md.
Install it in Ubuntu/Debian using:
| 1 | sudo aptitude install mb2md | 
To make the convertion use:
| 1 | mb2md -m | 
From man:
| 1 2 |        -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!