Best way to add custom / magic / flutter fields into RSS feed

you can use something like this

function feed_magic_fields( $content ) {
  global $post, $id;

  if ( !is_feed() )
    return $content;

  // is feed
  $date = get('date_event');
  if( $date)
    $content .= $date

  return $content; 
}

add_filter( 'the_content', 'feed_magic_fields' );