How to manually add an item to a submenu in a menu generated by wp_nav_menu?

the simplest way would be to create two menus on for admin and one for everyone else
and display them based on that:

if (current_user_can('add_users')){
   wp_nav_menu( array('menu' => 'My Admin Menu' ));
}else{
   wp_nav_menu( array('menu' => 'My Users Menu' ));
}