Yes you can do this...

for achieving this you'll need to configure storage lifecycle policies

for more information refer to NBU Admin Guide I, Storage units, unit groups, and lifecycle policies chapter

White Paper: Veritas NetBackup 6.5: Implementing Storage Lifecycle Policies

This screenshot might show the feature your looking for to perform the expire.
You might be able to script it within one of the notify commands that also run. But SLP is probably best option.

Temporary Storage Units Staging is another free option that is native to NetBackup.  It does not have the flexibility of SLP but works very well.

You can do this by going to the properties of each storage unit and turning on the Enable Temporary Staging Area option.  Under this you can set up a staging schedule for when to offload your data to alternative locations/media.  What this will do is duplicate completed images from disk to other destinations (offsite media) based on your schedule.

Based on your high and low water marks, NetBackup will remove only images that have already been duplicated as it requires the space so your images will be available on disk for as long as possible until NetBackup requires the space.

Cheers