After 10 years I upgraded my computer, not a gamer but there was a sale on a prebuilt that came with an decent GPU. So I downloaded some games (pirated). Really only play American Truck Simulator and Civilization VI. My question is that in order to play online (through Steam?) I need to actually buy the game so that it will show up on my steam account?

Steam is having a sale and American Truck Simulator is like $7 but the DLCs are really expensive! Can I use the DLCs that came with Pirated Games with the "legit" games you bought from Steam?