The simplest way to add a user as super admin is to have another super admin grant super admin privileges from WordPress dashboard. However, there may be times when this is not possible or is not convenient and you still need to add a super admin to your multi-site
Another easy way to assign an existing user with super admin privileges is to add the following function to your functions.php file.
grant_super_admin( $user_id );
You’ll need to replace the $user_id parameter with the user ID of the specific user you’d like to assign these privileges to. If you don’t want to have to dig up the user ID but know the email address of the user, you could use these two lines of code and replace ‘firstname.lastname@example.org’ with the email address of the user.
$new_super_admin = get_user_by_email( 'email@example.com' ); grant_super_admin( $new_super_admin->ID );
Once this function has been added, check to confirm that the user now has super admin privileges. If it was successful, remove the piece of code as it is no longer needed.