I don't believe that there is an official bacula mechanism for doing what you want.
This is what I did this for a customer with a small office (5 PCs, a server, and 2 servers on the internet).
First, I ran the nightly backup jobs, which backed up to the server's disk. Next, I ran a script that would restore to another location on disk (we have plenty of space) and then tar and feather to tape.
The reason I did this, as opposed to sweimann's good suggestion of simply backing up the backup files to tape, was that I wanted maximum portability of the final tapes. You can pop a tar'ed tape into any machine (even windows, w/ the right software) and restore the files.
The online backups are the true primary, and they go back for six months. The tapes, which were always limited to the most recent full restore, are mainly for CYA disaster recovery in case the office burned down (they were meant to be taken off site).
No, it doesn't scale terribly well, and it takes much longer than a direct backup, but it works for some scenarios.