class TestServiceProvider implements ServiceProviderInterface
{
    public function register(BaseApplication $app)
    {
        // 商品CSV登録（継承）
        $app->match('/' . $app['config']['admin_route'] . '/product/product_csv_upload',
                '\Plugin\Test\Controller\CsvImportControllerEx::csvProduct'
            )
            ->bind('admin_product_csv_import');
    }
}

<?php
namespace Plugin\Test\Controller;

use Eccube\Application;
use Eccube\Controller\Admin\Product\CsvImportController;

class CsvImportControllerEx extends CsvImportController
{
    /**
     * 商品登録CSVアップロード
     */
    public function csvProduct(Application $app, Request $request)
    {
        echo "OK";
        die;
        
        parent::csvProduct($app, $request);
    }
}