Help with Custom SignIn/SignUp flow

I need to set up a custom sign-in/sign up flow.

User sign-in/sign up through a custom OIDC Identity Provider. This identity provider is a proxy that I setup because the original idp - Valve’s Steam platform - still uses open id 2.0, and would be the only way to sign in at my website.

After I get the authorization from the IdP, I receive only a string that mentions the user’s account at Steam’s platform. Now back at my website, I need to ask the user for an email and then ask auth0 to verify it. What is the best way to do this?

Since there is no email claim from the IDP, there is no email parameter at the normalized user profile. How can I set a “blank” email, to be updated later through the management API?

Also, is one open id connect provider available in the free tier?