Custom field in XML with Views RSS module
You need to implement a hook_views_rss_item_elements() invocation if you're wanting to add a custom item element. For example:

/**
 * Implements hook_views_rss_item_elements().
 */
function MYMODULE_views_rss_item_elements() {
  $elements['my_element'] = array(
    'title' =&gt; t('My Element'),
    'description' =&gt; t('Provides a my_element to an item element in Views RSS.'),
  );
  return $elements;
}