What is the source of the tadpole? Harvesting of wild animals for the pet trade should not be condoned under any circumstances. You're correct, leopard frogs are not native to your area, and it would be totally irresponsible to release one there. You would be responsible for the frog, which means taking care of it properly, for its lifetime. Unless your frog becomes part of a captive breeding program for reintroduction purposes, you would be doing nothing to help the species by raising and keeping one in captivity; in fact, if it came from the wild, you would be doing the opposite.