Is it possible to map the GitHub Access Token into the Access Token provided to my app after login (given that the user has connected their GitHub Account and authorized my app)?
I tried to add an Identity Provider Mapper and Client Mapper:
You can use this to get the token, (assuming you’ve checked Store Tokens and Store Tokens Readable in the idp setup) Server Administration Guide
Thanks, is there also a way to retrieve the username?
Yes that is in the token as well. This is my function to get it: keycloak-react-demo/layout.tsx at 3dc146cb1a9304e36449d4d4d0506987760562d6 · edewit/keycloak-react-demo · GitHub
Sorry, I mean the GitHub Username 
That would be the github user name when you use a Identity Provider