Freezing would be good - in fact you might really WANT to do that to stave off whatever parasites they might pick up in the barn. I do know from experience mice REEK - I have heard rats are much less stanky.

Of course you could always order from rodentpro.com or another supplier and have a buncha frozen shipped to you (which is what we did once we realized the undertaking of breeding as many as we'd require).