Search additional user fields?
As far as I know, the easiest way to do this is with views. You can set up a user view with exposed filters for any user field that you need to search. If you want to get a link to your your view on the search page form then, a form alter in a custom module would do it. Obviously make sure your view has a page with a proper path setup. search/your_view etc will work. 

Here is a quick example of adding a custom markup field to the search form in drupal 7. 

function CUSTOMMODULENAME_form_alter(&amp;$form, $form_state, $form_id) {

 if ($form_id == 'search_form') {

   $form['usercustomsearchlink'] = array(
  '#type' =&gt; 'markup',
  '#markup' =&gt; '&lt;a href="http://YOURSITE.com/search/YOURVIEWNAME"&gt;Search User Fields&lt;/a&gt; ',
  '#weight' =&gt; 1,
   );

   }
}


That should put a link underneath the search box on the form (after a cache clear). However, I havent been able to figure out how to make it appear only on search/user pages yet so, that link will appear on all search forms.