From BcakupExec 12.0, there hasn't been the DPM backup agent any more. So you can no more backup your DPM directly from BackupExec. However, there is a workaround and it's relatively easy:

There is a built-in backup tool for DPM. It's named DpmBackup.exe and it's located in the folder: C drive(If you install DPM on C drive) -> Program Files -> Microsoft Data Protection Manager -> DPM -> bin. This built-in backup tool will backup all the replicas for the proteced servers and the DPM database to the folder: C drive(If you install DPM on C drive) -> Program Files -> Microsoft Data Protection Manager -> DPM -> Volumes -> ShadowCopy. it will create individual subfolders for each protected server and DPM database. if you need only backup your replica, you can use the command as DpmBackup.exe -replica; similarly, for DPM database only, the command is DpmBackup.exe -db.

You can then use the BackupExec and other VSS not supported backup software to backup the shadowcopy folder.

You can of course run the tool manually before the daily backup but obviously it's better to make it running automatically. the easiest way is to to use a script or batch file that launch the DpmBackup.exe and use the Windows scheduled tasks to run the script every day/weekday.