So guys, I just thought... I wanted to try the Steam MP of MW2 and see the diff. Problem is: 1) My copy of MW2 is pirated (meaning it was only self-extracted to the computer and not installed). 2) I don't know how to merge it. and 3) Steam is asking a keycode from me. I don't know my MW2's keycode or if there is any...

If it ask you for a key then that means you didn't buy the game for steam.

 

It only cost $20. not worth it imo

So you basically want to play on steam servers without buying to try it out? That won't happen unless they give you a free steam weekend, which they probably won't considering the game is old.