SampaioLeal you can insert users in the database via the Flarum API or directly via SQL. You should find some information on how to use the API by searching this forum. As far as I know there's no step-by-step guide for what you're trying to achieve though.
Regarding GSuite login I'm not sure you could limit the login to that one domain with the existing Google Login extension ?
I see you have some comments in the Google Login extension discussion. Let's try to keep comments there strictly related to the features of that extension and we can discuss here what other possibilities you have or how to import those users.