Login in to 3rd party API and saving some info

[enboig]

I have an app which extends AbstractFormLoginAuthenticator for normal users login. It sends a form with user and password, gets a token, and use it to get user permissions, roles, etc...
Now I want to add api services, I tried extending JWTTokenAuthenticator and also implementing AuthenticatorInterface but I couldn't intercept the login to use my custom code to login (and save the extradata somewhere, so I could reload it in UserCheckerInterface.
My security.yaml looks has:

    firewalls:
        api_login:
            pattern:  ^/api/login
            # user_checker: App\Security\MyUserChecker
            stateless: true
            anonymous: true
            json_login: # or form_login
                provider:                 app_user_provider    #or your custom user provider 
                check_path:               /api/login_check  #same as the configured route
                success_handler:          lexik_jwt_authentication.handler.authentication_success
                failure_handler:          lexik_jwt_authentication.handler.authentication_failure
                require_previous_session: false
                username_path: username
                password_path: password
            guard:
                authenticators:
                    # - lexik_jwt_authentication.jwt_token_authenticator
                    - App\Security\ApiTokenAuthenticator
                provider:                 app_user_provider    #or your custom user provider 
        api:
            pattern:   ^/api/v1              # protected path
            stateless: true
            guard:
                authenticators:
                    # - lexik_jwt_authentication.jwt_token_authenticator
                    - App\Security\ApiTokenAuthenticator
                provider:                 app_user_provider    #or your custom user provider 

I suppose I should modify json_login; but I could find how.

I am using symfony 4.4 currently; I found #372 but it's an old entry and hasn't a solution.