I'm sorry Gorally, I misunderstood you.
I don't know a way to manually change the state of a file from inactive to active but...........
Once the restore completes, wouldn't the state automatically be changed to active the next time a bckup runs?
If you restore the file, and the restored file has the exact same date, size location etc... as the latest inactive file, I would think that the latest inactive file would automatically be changed to active and that the file wouldn't be backed up again.
I have never actually tried this before, I am only going by an educated guess with past expreiences with TSM. I am not sure about it.
The alternative would be that the file would be backed up again, even though an exact duplicate of the file is already inactive in the database.
Gorally, I'm just not sure either way. PLEASE keep us posted as to what will happen!