How to link a Custom Post Type submenu to a Nextgen Gallery page?

It would go like this (note the absence of the function parameter – and also the capability instead of a role).

add_action( 'admin_menu', 'wpse_74421_menu_admin' );

function wpse_74421_menu_admin() 
{
    add_submenu_page( 
        'edit.php?post_type=events', 
        'Add Gallery', 
        'Add Gallery', 
        'delete_plugins', 
        'admin.php?page=nggallery-add-gallery'
    );
}