Its very simple to attach image from any external url to post by code.
Here is the simple code that will attach image to your post.

set_time_limit(300);
require_once(ABSPATH . 'wp-admin/includes/file.php');
require_once(ABSPATH . 'wp-admin/includes/media.php');
require_once(ABSPATH . 'wp-admin/includes/image.php');
$upload = media_sideload_image('IMAGE_URL', "POST_ID", "IMAGE_DESC");
die ( $upload );