code: public function appendChoice(FormEvent $event)
      {
          $rootForm = $this->getRootForm($event->getForm());
          $group = $rootForm->getData();
          
          if ($group === NULL) {
              return;
          }
  
          $choices = array();
          foreach($group->getCustomFields() as $customFields) {
              $choices[$customFields->getSlug()] = 
                      $this->translatableStringHelper
                          ->localize($customFields->getName());
          }
          
          $options = array_merge($this->options, array(
                  'choice_list' => new SimpleChoiceList($choices),
              ));
          
          $event->getForm()->getParent()->add($this->choiceName, 'choice', 
                  $options);
      }
docs: append Choice on POST_SET_DATA event
  
  Choices are extracted from custom_field_group (the data associated
  with the root form)
  
  @param FormEvent $event
  @return void