Backup Exec does not have support for what you are requesting. A backup job relies on two factors: the device to backup to and the media pool which the backup will be stored on. You cannot configure Backup Exec to always run a particular backup to a certain slot in the robotic library.
It would perhaps be possible to designate a certain pool of tapes which will be used for this backup, and then define these a separate media set in Backup Exec. You may then be able to get success with what you intend by assigning the particular job to this new media set.
-Matt
