As I understand it, there’s a way to activate/register a game via steam. I think the option is somewhere under the ‘file’ menu (I’m at work and cant check right now). You should just be able to put you key in there if it’s using the same registration system. I’ll try it when I get home since I’m going to be reformatting my computer with weekend anywho.
I’m pretty sure that only works with a few of valves games. Also cliffski would say it steam allowed to use the registration function.