jmshorb the user "me" endpoint configured with the "Api URL providing user details when authenticated" setting is used as the default registration payload. If you return values that match the Flarum json:api attribute names, then Flarum should use those values and not prompt the user. The attribute names should be username and nickname, directly at the root of the JSON object.
I've never tried with the nickname attribute, but I expect it's also supported through this feature. The actual underlying feature that powers this is part of Flarum. The extension just uses setPayload on the registration state object, pushing the entire Passport API response as the content.