You can add non-steam games to steam if thats what you mean. Theres a launcher called hydra which does, things. Playnite is a launcher I use that I plug all my other accounts in to and launch all games thru that. There are also ways to unlock dlc and stuff but that will require a little bit of research. Havent sailed the seas for games in a while.

TLDR; Yes you can add otherwise aquired games to steam. For the most part