Add Site Admins through cfg.php tool
You can use the site configuration
admin/cli/cfg.php tool to add a site admin if you know the user's Moodle user id.
Here's a quick example
php admin/cli/cfg.php | grep siteadmins siteadmins 2,669,2104,3328,4350
This gives us the user ids that are currently siteadmins (e.g id 2 is the admin user).
Now lets add the user id,
5000 to this list, making sure that we retain the the other ids (otherwise they will lose site admin!).
php admin/cli/cfg.php --name=siteadmins --set=2,669,2104,3328,4350,5000
Then verify the list again:
php admin/cli/cfg.php | grep siteadmins siteadmins 2,669,2104,3328,4350,5000