You should be able to do this (untested, but I think it'd work).  Update the
device class that points to the library to point to the manual library
instead. Then do your restore, the server will prompt you for the tapes
(you'll have to do q req and do some replies I think).  Then, when your
restore is done, update the device class to point to the automated library
again.

Just make sure that you don't have any other operations that use that
devclass happening while you've got it pointed to the wrong library....

Paul