This is quite easy to do.  Here is how you would setup your storage pools
in the system:

 disk-pool-standard     disk-pool-new
                \       /
                 \     /
                tape-pool

Upon registering a user, you could assign them to the "new" policy domain,
where the default copy destination for backup files is to the "disk-pool-new"
storage pool.  After a week, you could issue
   UPDATE NODE nnnn DOMAIN=STANDARD
to assign that node to the standard domain.

In the new policy domain you should define the same mgmt. classes as exist
in the standard policy domain just so users have the same policies for
managing their data.  Once the user is moved to the new policy domain, all
pointers to data are still valid and maintained by the system thus restores
will be just fine.

Hope this helps,

Paul Bradshaw