is it possible to edit default output of services module?
Using the Services API this is possible: http://cgit.drupalcode.org/services/tree/docs/services.alter.api.php?h=7.x-3.x

/**
 * Implements hook_services_request_postprocess_alter().
 */
function example_services_request_postprocess_alter($controller, $args, &amp;$result) {
  switch ($controller['callback']) {
    case '_node_resource_retrieve':
      $result-&gt;foo = 'bar';
      break;
    case '_user_resource_retrieve':
      $result-&gt;bar = 'chew';
      break;
  }
}


You could use the Views JSON module (a sub module of the Views datasource module) to build a View with a User ID contextual filter, and then return the field(s) you want for that particular user.