FormEvents :: POST_SUBMIT中更新的数据不会发送回控制器

Question

I am working with Symfony 2.7 and have created a custom FormType based on Symfonys EntityType. Within the FormEvents::POST_SUBMIT of this type I update the received data by adding a special entity. However this chance is not send back to the controller:

Action within controller:

public function customEditAction(Request $request) {
    $formData = $this->getFormData();
    // formData has a property myEntities which is array of MyEntity

    $form = $this->createForm('my_form_type', $formData);
    $form->handleRequest($request);

    if ($form->isValid()) {
        foreach ($formData->getMyEntities() as $myEntity)
            dump($myEntity);
    }

    ...
}

my_form_type

class MyFormType extends AbstractType {
    ...

    public function getName() {
        return 'my_form_type';
    }  

    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder
            ->add('myEntities', 'my_entities_type', array(
                ...
            ),   
        ));

        ...
    }
}

my_entities_type

class MyEntitiesType extends AbstractType {
    ...
    public function getParent() {
        return 'entity';
    }

    public function getName() {
        return 'my_entities_type';
    }  

    public function buildView(FormView $view, FormInterface $form, array $options) { 
        ...
    }  

    public function buildForm(FormBuilderInterface $builder, array $options) {
        $builder->addEventListener(FormEvents::POST_SUBMIT, function (FormEvent $event) use ($options) {
            // Update data with a special entity.
            $data = $event->getData();

            $specialEntity = $this->getSomeSpecialEntity();
            $data[] = $specialEntity;

            // Dump shows, that the entiy was correctly added to $data
            foreach ($data as $myEntity)
                dump($myEntity);

            $event->setData($data);
        }
        ...
    }
}

Expected Result:

1. Form is created with some set of entities within $formData->getMyEntities()
2. Form is presented to the user. EntityType (parent of MyEntitiesType) fetched all available Entities to build a selection. Entities within $formData->getMyEntities() are pre-selected
3. User makes his choice and selects a new set of Entities, e.g. Entity1 and Entity2
4. Form is submitted
5. In FormEvents::POST_SUBMIT $specialEntity is added to the data/selection
6. Both dumps (within POST_SUBMIT and within the controller) should show the selected entities including $specialEntity

Everything works fine beside step 6: While the dump within POST_SUBMIT shows that $specialEntity is in $data, the dump within the controller only shows the user selected entities...

Why isn't the changed data submitted back to the controller? What do I have to do, to get the updated data to the controller?

EDIT:

Adding the special entity in FormEvents::SUBMIT does not work, since this entity is not managed and will thus result in an exception within the EntityType implementation.