You can add a non-steam game if it is “installed” (read extracted), then in steam select to use compatibility mode in settings for the game. That will run it with proton and has a very high sucess rate. So far think i came across 2 that did not work. One of the civ games and another. Ironically both i actually bought rather than pirate, but if you install a game via wine, you can add it via steam. Actually i do this with a bunch of legally bought games as its an easy way to manage a library.