function add_featured_image_url($output) {
    global $post;
    if ( has_post_thumbnail( $post->ID ) ){
        $output = wp_get_attachment_url( get_post_thumbnail_id( $post->ID ) ) . \' \' . $output;
    }
    return $output;
}

add_filter(\'the_excerpt_rss\', \'add_featured_image_url\');
add_filter(\'the_content_feed\', \'add_featured_image_url\');