--------- config.yml ----------
name: カスタム特定商取引法
version: 1.0.0
code: CustomTradelaw
event: Event

-------- event.yml --------
admin.setting.shop.trade.law.index.initialize:
    - [onCostomTradelawInt, NORMAL]

-------- Event.php --------
namespace Plugin\CostomTradelaw;

use Eccube\Event\EventArgs;

class Event
{
    private $app;

    public function __construct($app)
    {
        $this->app = $app;
    }
    /**
     * メールアドレスの必須を除外
     * @param EventArgs $event
     */
    public function onCostomTradelawInt(EventArgs $event) {
        $builder = $event->getArgument('builder');
        $builder->add('law_email', 'email', array(
                'label' => 'test',
                'required' => false,
                'constraints' => array(
                    new Assert\Email(array('strict' => true)),
                    new Assert\Regex(array(
                        'pattern' => '/^[[:graph:][:space:]]+$/i',
                        'message' => 'form.type.graph.invalid',
                    )),
            ),
        ));
    }
}

--------- config.yml ----------
name: カスタム特定商取引法
version: 1.0.0
code: CustomTradelaw
event: Event

-------- event.yml --------
admin.setting.shop.trade.law.index.initialize:
    - [onCostomTradelawInt, NORMAL]

-------- Event.php --------
namespace Plugin\CostomTradelaw;

use Eccube\Event\EventArgs;

class Event
{
    private $app;

    public function __construct($app)
    {
        $this->app = $app;
    }

    public function onCostomTradelawInt(EventArgs $event) {
        $builder = $event->getArgument('builder');
        $builder->add(
            'plg_test',
            'text',
            array(
                'label' => 'テスト項目',
                'mapped' => false,
            )
        );
    }
}