Custom Register/Login Structure

I would like to implement a custom login and registration structure but am not really sure where to get started on the wordpress end.

I would like to have users simply login through the Steam Community OpenID, if they are not currently a user, to set them up and log them in.

The Steam Community OpenID works a bit differently where the only data returned on a successful login is the user’s unique SteamID. I already have code that directs a user to the login page, on successful login retrieves the steam id, and converts that steam id in order to retrieve some of their public information. This all works fine. I’m just not sure how to tie this into wordpress.

https://steamcommunity.com/dev/
https://developer.valvesoftware.com/wiki/Steam_Web_API#GetPlayerSummaries_.28v0002.29 (I’m using GetPlayerSummaries to access public info based on SteamID).

So my initial thought was to map information to the proper spots and I’d be good to go. But obviously I shot down that thought quickly because WordPress registration needs a email and password. I don’t want to use either of those pieces of information, nor do I have access to information like that.

So only armed with a unique SteamID, and a PersonaName (which I could map to the Display Name). How would I look up to see if the user is in the system? If they are not, register them without them having to activate the account (should just activate because I trust they are a real user at this point)? Log them in if they are in the system?