How to add post featured image to RSS item tag?

You could do it by adding an action to the hook ‘rss2_item’ like so:

add_action('rss2_item', function(){
  global $post;

  $output="";
  $thumbnail_ID = get_post_thumbnail_id( $post->ID );
  $thumbnail = wp_get_attachment_image_src($thumbnail_ID, 'thumbnail');
  $output .= '<post-thumbnail>';
    $output .= '<url>'. $thumbnail[0] .'</url>';
    $output .= '<width>'. $thumbnail[1] .'</width>';
    $output .= '<height>'. $thumbnail[2] .'</height>';
    $output .= '</post-thumbnail>';

  echo $output;
});