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.
Bitcoin itself is detached from any other money system. Any form of "converting" BTC to other currencies is basically an agreement with another person in which you send them bitcoins, and in return they send you some other currency using a method you specify. So the answer to your question depends solely on which kinds of agreement you manage to reach with other people.
Bitcoin exchanges are basically an organized way to make such agreements, and getting your funds depends on the withdrawal methods offered by the exchange. Some of the offered methods do not require a bank account.
The question about Steam Wallet is more about Steam Wallet than about Bitcoin, and the answer depends on the technical and contractual feasibility of transferring funds to another person's Steam Wallet. According to ThePiachu's answer, this is impossible.