altering user_admin_account form
I faced a similar issue while adding a new field 'Mail' in the 'user_admin_account' form. 

During my search I came across this post &amp; also this post 'https://drupal.org/node/1899708' which address the same issue. I got a solution for this as described below:


/**
 * Implements hook_form_FORMID_alter()
 * @param $form
 * @param $form_state
 * @param $form_id
 * @return $form 
 */
function CUSTOMMODULE_form_user_admin_account_alter(&$form, &$form_state, $form_id) {
  // Adding User Mail Field in the 'user_admin_account' form
  $mail_header = array(
    'mail' => array('data' => t('Mail'), 'field' => 'u.mail'),
  );
  // Placing Mail at 2nd position
  array_splice($form['accounts']['#header'], 1, 0, $mail_header);

  foreach($form['accounts']['#options'] as $key =>$value){
       $query = db_select('users', 'u')
        ->fields('u', array('mail'))
        ->condition('u.uid', $key)
        ->execute()
    ->fetchAssoc();

    array_unshift($form['accounts']['#options'][$key], $query['mail']);
  }

  $form['pager'] = array('#markup' => theme('pager'));

  return $form;
}


Although this post dates back in '11, but as i did not get any solution based on my search so considered to post this answer. Hope it helps.