Yeah, the user created NP Bank was a flop. I don't think there's any legitimate way you can earn interest on those NP$. Might as well spend them all on domains and thread highlights.
Members are welcome to loan each other money by whatever agreements they wish to in private. If you want to offer loans to people with terms you set, go ahead at your own risk. I don't think it's a good idea to call yourself a bank though.