Add an extra field to a content type display

Hook_node_view() can be used to add the block to the renderable $node->content array.
If the weight/order of the extra field needs to be set on the manage display tabs from your article content type, use hook_field_extra_fields() to tell Drupal about the new field.

/**
 * Implements hook_field_extra_fields().
 */
function MYMODULE_field_extra_fields() {
  $extra['node']['article'] = array (
    'display' => array(
      'extra_article_field' => array(
        'label' => t('Extra article field'),
        'description' => t('add an extra article field to the article content type.'),
        'weight' => -99,
      )
    )
  );

  return $extra;
}

/**
 * Implements hook_node_view().
 */
function MYMODULE_node_view($node, $view_mode, $langcode) {
  switch($node->type) {
    case 'article':
      $node->content['extra_article_field'] = array('#markup' => 'Use a form or theme function here.');
      break;
  }  
}