1. A Bethesda.net account and an ESO Account are two separate accounts. Since you're already playing ESO I'm going to guess you are playing with an ESO Account It is possible for you to make a purchase of a Summerset Upgrade to your current account on Steam only if your game account is linked to Steam. In order for your game account to be linked to Steam you must have a Steam purchase of ESO.
2. Yes, it is possible to buy Summerset plus the basegame and link it to your current account. As long as Steam recognizes that you have a purchase of ESO, you can choose your current game account to link to your Steam account. You can check the above link for a detailed step-by-step.
Steam's services does not provide game keys for any of our games.
They could do it again, for the convenience of their players, who pay them a LOT of money for this game, and for their devs, who live off that money we pay them.
Taleof2Cities wrote: »They could do it again, for the convenience of their players, who pay them a LOT of money for this game, and for their devs, who live off that money we pay them.
Not all players prefer the Steam overlay, @MrKit.
I don't.
I'd rather launch the game from the game launcher.
Fortunately for players like me, this is ESO ... and not Fallout, Sim City, or Planet Coaster.