Steam question

I was thinking of buying Guild Wars: Nightfall through steam, problem is I already have an active Guild Wars account that isn't linked to it. I was wondering, if I buy the Nightfall stand-alone campaign, through steam, would I be able to add it to my active Guild Wars account, which is not linked to steam?

When you buy the game on Steam, you are given an access code. That is really the only thing you need, and it's no different than receiving the code from a retail box.

All you do is take that Nightfall key and add it to your existing Guild Wars account through the "Edit Account" button in the character selection screen.