Short of using OAuth2, you could write your own app component to tie NodeBB into MySQL. One of the issues is that MySQL is a generic database so the format of your authentication easily won't match in some obvious way, so needs a customer connectors no matter what. Just adding another DB source for authentication would not solve your problem. It would have to be something that connects to your database and has the ability to understand your schema and has the ability to store all necessary additional schema changes there.
