hi
how can I convert my arma2 co from gog to steam version ?

thanks

I have Arrowhead installed on steam, but cd key is not working. so I think about putting gog cd key into it.. do u think it will work?

or how just convert gog key to steam key ..because I dont have A2 on steam

What you are asking for is impossible.
You CAN install ARMA 2 anyway with this:
Click on this link - steam://install/107400

That will install the ARMA 2 base game on to Steam, but you can't add Operation Arrowhead that way, so no DayZ via steam or whatever you are looking for. You can just install both versions as GoG, ad it as a non-Steam game if you want the overlay (or just Try Raptr perhaps instead), and use something like DayZ commander if you want that mod.