That Steam license allowing you to launch FFXIV and use Steam overlay, the Product Key given by Steam can be used by anyone, as long as they get the FFXIV game client directly from http://www.finalfantasyxiv.com/playersdownload/na/

So you can use the Steam copy, to launch the game, and log in using your existing SE Account/Service Account. As mentioned earlier, for non-CE Windows to be upgraded to CE, add PS3/PS4 CE license to your PSN/SEN which will be linked to a SE account and make sure you link to the right Service Account. Or upgrade in Mogstations.