$form = $app['form.factory']
        ->createBuilder('order', $TargetOrder)
        ->getForm();

use Symfony\Component\Validator\Constraints as Assert;

    ･･･

    $builder = $app['form.factory']->createBuilder('order', $TargetOrder);
    $builder = $this->buildFormForPointPlg($builder);
    $form = $builder->getForm();

    ･･･

    private function buildFormForPoint($builder) {
        $builder->add(
            'use_point',
            'integer',
            array(
                'label' => '利用ポイント',
                'required' => false,
                'mapped' => false,
                'attr' => array(
                    'class' => 'form-control',
                ),
                'constraints' => array(
                    new Assert\GreaterThanOrEqual(array('value' => 0)),
                    new Assert\Length(
                        array(
                            'max' => $this->app['config']['int_len'],
                        )
                    ),
                ),
            )
        )->add(
            'add_point',
            'integer',
            array(
                'label' => '加算ポイント',
                'required' => false,
                'mapped' => false,
                'attr' => array(
                    'class' => 'form-control',
                ),
                'constraints' => array(
                    new Assert\GreaterThanOrEqual(array('value' => 0)),
                    new Assert\Length(
                        array(
                            'max' => $this->app['config']['int_len'],
                        )
                    ),
                ),
            )
        );

        return $builder;
    }