I've never done it, but for EXISTING data I THINK you could: 

1) EXPORT the nodes and specify FILEDATA=ARCHIVE
That will put just the archive data on a tape.
2) Run DELETE FILESPACE for everyfile space and specify TYPE=ARCHIVE.
That will get rid of the archive data from the tape pool

That may or may not free a lot of tapes in the short term, but after RECLAIM
runs you should get the space back.

That takes a LOT of effort, and you no longer have a record of what was
archived since the DELETE FILESPACE takes it out of the data base.

Then set up separate pool for archives, and change your management classes
to point there.

Then I THINK you can even IMPORT the archives you have just exported (if you
do it BEFORE any new archives are created) and that will force the data into
the new pool...but I'm not positive about that, you'll have to test it.