Add custom element into Wordpress RSS feeds

functions.php

add_action('atom_entry', 'add_thumbnail_to_feed');
add_action('rdf_item', 'add_thumbnail_to_feed');
add_action('rss_item', 'add_thumbnail_to_feed');
add_action('rss2_item', 'add_thumbnail_to_feed');
function add_thumbnail_to_feed(){
    global $post;
    if ( has_post_thumbnail( $post->ID ) ){
        $thumb = wp_get_attachment_image_src( get_post_thumbnail_id($post->ID), 'thumbnail' );
        $url = $thumb['0'];
        echo "<thumbnail>$url</thumbnail>";
    }
}