You just have to switch your account to a steam account. I did that a while ago so that my wife could get me steam gift cards. I do remember how to do it though. I imagine a quick search on the internet would give you the answer.
SeaGtGruff wrote: »I think if you switch your account, it will be like starting over and you'd need to rebuy everything-- but I don't know, because the only way I've ever bought and played ESO is through Steam.
However, your characters and purchases are linked to your account, so it seems like switching to a different account in ESO is going to give you a clean slate as it were.