Insert the URL of the page as the $menu_slug
argument. Also note that user levels are deprecated, you should pass a capability instead.
function add_custom_link() {
add_submenu_page(
'edit.php?post_type=cpt_custom',
'',
'Pending Posts',
'edit_posts',
'edit.php?post_type=cpt_custom&post_status=pending',
''
);
}
add_action('admin_menu', 'add_custom_link');