Google Auth for pre enrolled users in only one step

I’d like to get rid of the 2nd step to use the Google Auth Plug in (1- Admin registers the users with local pass. 2- make each user click on “Link with google”).

Why not just allow the pre-registered users to log in with Google Auth without facing the second step?

As far as I know this is working with “Allow Account Creation”, but not for the pre enrolled users who have their proiects already assigned and possibly tasks too.
Leaving the “Allow Account Creation” open is not an option for security reasons.