Several methods really.

First for $superadmin to take effect you have to remove the // in front, otherwise it's handled as a comment.

But you could even simply add your alts to your main and then use shared security-settings over alts (it's a security setting). That way you only need to insert the main of all wanted super-admins.

Otherwise I think !settings is as default only available to superadmins and owners, so you'd have to add the characters to the group super-admin, not admin.