Possums are the only marsupial native to the Americas, they are remarkably clean animals, immune to rabies, and can eat just about anything. They are not predators, but they are opportunists. I'd just set out more food, and let it clean up the barn for me. They're not vermin, like most people think. They'll hang out and be relatively friendly if you are too.
[ame]http://www.youtube.com/watch?v=kCTx6U10Jck[/ame]
[ame]http://www.youtube.com/watch?v=kCTx6U10Jck[/ame]