So Imperial Edition and the DLC all link to your account. Once you link your existing ESO account with your Steam account, you should have access to the Imperial Edition on Steam and the DLC you bought with the Gold Edition.
However, if you go back and login to your ESO account through the game launcher (i.e. access your game without using Steam like you have in the past) I am not sure that the Gold Edition will be available. It might be, but I've just never tested this out.