adding Extended fields from snippet using security/user/create processor

$response = $modx->runProcessor('security/user/create',array(
   'username' => $hook->getValue('firstname').$hook->getValue('surname'),
    'fullname' => $hook->getValue('firstname').' '.$hook->getValue('surname'),
    'address' => $hook->getValue('address1'),
    'county' => $hook->getValue('address2'),
    'city' => $hook->getValue('address3'),
    'zip' => $hook->getValue('postcode'),
    'phone' => $hook->getValue('phonenum'),
    'mobilephone' => $hook->getValue('mobilenum'),
    'email' => $hook->getValue('email'),
    'newpassword' => TRUE,
    'passwordnotifymethod' => 'e',
    'passwordgenmethod' => 'g',
));
if ($response->isError()) {
   if ($response->hasFieldErrors()) {
       $fieldErrors = $response->getAllErrors();
       $errorMessage = implode("\n",$fieldErrors);
   } else {
       $errorMessage = 'An error occurred: '.$response->getMessage();
   }
   $hook->addError('error_message',$errorMessage);
return false;
}
return true;

$response = $modx->runProcessor('security/user/create',array(
   'username' => $hook->getValue('firstname').$hook->getValue('surname'),
   /* ... */
   'extended' => array(
      'fieldname' => 'value', /* etc */
   ),
));