No. Buying a game on one platform doesn't give it to you on any other. Unless the game came with a steam key there is no way.

You can only add them to your library as non-Steam games. Only games purchased on Steam or keys that are redeemable on here can be added to your account.

Originally posted by navymom0257:

If Steam has games that I purchased several years ago, is there a way to add said games to my library without purchasing them on Steam? For example, I bought the adelantado series several years ago and I see that Steam is offering them now.

Do they have CD-Keys? If so, you can try them, though don't expect them to work. Only some key worked to get the game on Steam.

Other then that, it is as has been said. Buy them on Steam or add them as a non-steam game.

Purchases on one store do not carry offer to other stores.

If you want a game on Steam from any other place, you have to purchase Steam keys for that game. Third-party sites that sell games offer Steam keys. If you mean physical versions of game, you still have to have Steam keys to activate it on Steam. There is an option to add non-Steam games, You will just need to do Google search on each game you want to add to see if you can.