How can I add external javascript to aggregation?

How can I add external javascript to aggregation? What wrong I may be doing? Is there any other way?

I tried following in template.php with no luck

function _phptemplate_variables($hook, $vars) {
  if ($hook == 'page') {
    // Add the script.js file
    drupal_add_js(path_to_theme() .'/gn.js');
    // Reload the scripts variable to include the added file
    $vars['scripts'] = drupal_get_js();
    if(module_exists('javascript_aggregator') && $vars['scripts']) {
      $vars['scripts'] = javascript_aggregator_cache($vars['scripts']);
    }

    return $vars;
  }
  return array();
}