How to modify/add addition variable to service json after successfully register.json?
Add custom code/module to modify the service response, check hook_services_endpoint_response_alter and all other similar alter hooks services alter hooks

e.g.:

function MODULENAME_services_request_postprocess_alter($controller, $args, &amp;$result) {
  if ($controller-&gt;callback == '_user_resource_create') {
    if ($result-&gt;uid) {
      $user = user_load($result-&gt;uid);
      $result-&gt;user = $user;
    }
  }
}