It is possible to bypass OpenID login by providing a custom provider

Even if a wiki has an OpenID provider configured through its xwiki.properties, it is possible to provide a third party provider by providing its details through request parameters. One can then bypass the XWiki authentication altogether by specifying its own provider through the oidc.endpoint.* request parameters (or by using an XWiki-based OpenID provider with oidc.xwikiprovider.

With the same approach, one could also provide a specific group mapping through oidc.groups.mapping that would make his user automatically part of the XWikiAdminGroup