You must buy it again and add the Steam key on the Mog Station (you won't get the 30 days, tho, as those are for new accounts). It can't be done the other way around.

Quote Originally Posted by SE Kahuna
Nope, there isn't. Imagine it from Steam's perspective. You would register a game you haven't bought through them, that made them no money, but use their infrastructure to download a 10GB client, presumably more than once. Contrary to popular belief, server space and traffic come at a price
http://www.reddit.com/r/ffxiv/commen..._steam/cfi891z