If the STK tapes and the new IBM tapes are of the same media, one method is to use VM:Backup's tape copy utility, VMBTCPY, to copy these VM:Archive tapes. That's the simplest approach. The tapes (source and target) must be the same media though, as VMBTCPY does not support mixed media.
If, however, you are copying from one media to another, like 3480 to 3490, the recommendation is to run VM:Archive's Merge/Purge/Copy (MPC)
and define your output volume as a new VTS volser.