0o0kuki0o0 it certainly is possible, the amount of work will depend on the kind of system you are dealing with.
If your authentication platform supports Laravel Passport, or if you are able to write Passport-compatible endpoints, you could use https://discuss.flarum.org/d/5203-fof-passport-the-laravel-passport-oauth-extension
If the platform supports oauth or something similar to oauth, you could write a League OAuth driver to use with the FoF OAuth extension.
The other existing options, short of writing something from scratch, are the JWT-based solutions like https://discuss.flarum.org/d/21666-php-and-wordpress-single-sign-on-sso-with-optional-jwt-addon and https://discuss.flarum.org/d/30632-json-web-token-cookie-login
Those last 2 extensions might also be good examples if you want to implement something completely custom.
