Can I convert BTC to USD without a bank account? Just asking. Say, I want to switch BTC to USD inside Steam Wallet. Is it possible?
To my knowledge, Steam does not support BTC (at least yet). Moreover, Steam does not allow one to send another user money transfers, so you can't really exchange your funds with other people. You could, however, purchase a game gift for another person and sell that gift for Bitcoins, but that would probably be against Steam's policy.

If you generally want to exchange Bitcoins for any currency without having a bank account, you could always try finding another user that also wants to trade like that and exchange money with them, either in person or through services like Western Union. This way however, is less reliable than using a Bitcoin exchange.