Quote Originally Posted by frostmagemari View Post
Actually, maybe i misunderstood.. I wasn't thinking of buying just the expansions. I was meaning that i would rebuy the base game on steam, and all expansions and then log in using my current username.
The answer doesn't change. The regular Windows and Steam versions cannot co-exist on the same service account. Period. You would have to create a new service account or even a new Square Enix ID just to register it and pay a whole separate sub just for it, if I remember correctly.

It's a major part of the reason why everyone panicked when they implemented the client check depending on what license you had (Steam client for Steam license, Windows client for Windows license).