$builder->add('item0', 'choice', array(
                'label' => '拡張',
                'required' => true,
                'choices' => array('__unselected' => '選択してください') + ['1', '2'],
            ));

             $app['eccube.service.cart']->addProduct($addCartData['product_class_id'], $addCartData['quantity'],  $addCartData['item0'])->save();

    addProduct()内
        $this->setProductQuantity($productClassId, $quantity, $Choice);

    setProductQuantity()内
        if (!is_null($Choice)) {
            $CartItem->setChoice($Choice);
        }

    private $Choice;
    public function setChoice($Choice)
    {
        $this->Choice = $Choice;
        return $this;
    }

        $Cart = $app['eccube.service.cart']->getCart();

         {% set Choice = CartItem.Choice %}