The simplistic answer is, you can't use copy pools to keep data ONLY offsite longer.
What ever is in the Primary Pool will be in the Copy Pool. STG used onsite = STG used offsite.
Copy pools are intended as a method of recovering from lost or damaged primary media.
Converse to this, IF TSM could do this, and an offsite tape went bad, you've lost that data. Copy Pools provide for media "mirroring".
One other thing to keep in mind is that retention is handled by MGMTCLASSES and to STGPOOLS.