FSY generally using an iframe to embed the full Flarum application will result in poorer experience and bad mobile responsiveness.
Integrating the Flarum app and integrating the Flarum login are two different aspects which can be considered separately.
You can perform an initial import of your users via database import, but to keep the list of accounts in sync you will need a Flarum extension to integrate third-party login. Or a more advanced extension to use some sort of global login for both applications.
What is your main website built with? If it's Wordpress, there are two existing extensions to integrate login. If it's built with Laravel or other common frameworks that offer oauth2 server compatibility, you might be able to use the Passport Login extension for Flarum to integrate with an oauth2 server.
