Yes, that's normally what I did for those images which the end-user request for expiry extension. The only concern is that you have to find the correct backup id.
bpexpdate only runs against the NetBackup database. The physical location of the tape does not matter. This is true for expiring or recalculating image retention.
The bpexpdate command can cause catalogue corruption resulting in a loss of data if the image whose retention we are suppossed to modify has two copies of itself.
A workaround to the problem is to use the bpexpdate once more with the -copy < copy_number> switch next to the bpexpdate command being fired for the first time.