If you're not able to find an answer in the docs or issues, creating a new issue for the question may be the most helpful thing you can do for maintainers and other users:
jupyterhub/the-littlest-jupyterhub
jupyterhub/jupyterhub
jupyterhub/oauthenticator
I have no idea (and thus maybe shouldn't be talking),
but is there a way to associate a field of the original user with the GitHub user?
Is this relevant?
* "c.Authenticator.username_map doesn't work with local user"
Reframing the question as an issue title and/or description:
(How) Can I connect an existing PAM admin account to an OAuth authenticator account?
Can I stack authenticators in some way? E.g. create a compound authenticator that tries OAuth and then PAM, or PAM and then OAuth?