I was wondering because I have Half-Life and Half-Life 2 on discs and don't wont to buy something I already have.
I wouldn't know if its even possible. why would you need the steam copies if you have the original discs?
If you have the activation codes (CD keys) then yes, you can. If not... you can try sending valve a picture of your discs and asking nicely
I'm pretty sure to connect to any good servers on them you need steam. Oh plus I want dem acheivements :3
Isn't there a way for steam to detect a game? At library press at the bottom left corner "add a game" and then activate a product on steam.