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You can upgrade your steam account by adding $5. It doesn't matter what you do with that afterwards...

just.. wanna drop this here
https://www.steamgifts.com/giveaway/o9PpQ/dungeon-defenders

Yes i think you can

https://support.steampowered.com/kb_article.php?ref=3330-IAGK-7663

Some examples of purchases that grant access to these features are:

Adding the equivalent of $5 USD or more to your Steam Wallet
Purchasing game(s) that are equal to $5 USD or more from the Steam store
Adding a Steam Wallet card that is equal to $5 USD or more to your Steam account
Purchasing a Steam gift that is equal to $5 USD or more from the Steam store (Receiving a Steam gift from a friend doesn't count)

So you don't need to buy ANYTHING. You only need to ADD $5 to STEAM WALLET.

If this is not the answer, then I do not know what you're asking.

Some games allow you to buy stuff with steam wallet cash... Some don't. Can't give a list on which do or don't though, prob best to google answers.