Recover data from archived bkf

I manage a SBS2003 domain and we use the built in exchange server. I use Symantec BackupExec. I archived old backups and now I need to restore a mailbox from one of the archives. Since BackupExec already overwrote the media set for the archived backups, I can’t restore from the bkf in BackupExec. Does anyone know how I can extract the data from these bkf’s?