1. You already have an account for ESO?
2. You want to buy ESO on steam so you can play it through Steam?
3. You want to know if you can still use the same account on the steam version of the game?
Right?
If so, then yes, Steam is just a store front with added extras, when it launches ESO it will load into the same login screen you use now, just put your current account details in at the login screen and you can play with your old account.
However, to save money you could just link your existing ESO client as a non-steam game so you can launch it via steam with all the steam features instead of buying it again.
Ah ok, well you can use the same account via steam, but purchasing crowns etc through steam I have no idea about, I would assume you can do that, but I can't be sure.