Ability to make user site-admin by adding group to <site-admin> after migrated organization was switched to the centralized user management

Currently after organization was switched to the centralized user management from original user management, user can't be site admin by adding to <site-admin> group.

So it needs to add from Directory > User details > (...) > Assign site admin role

The behavior confuses customers. so please available to make user site-admin by adding group to <site-admin>