Using the addAttribute method for the ListProduct class

gistfile1.txt

class ListProductService implements ListProductServiceInterface
{

    /**
     * @var ListProductServiceInterface
     */
    private $service;

    /** @var Container */
    private $container;

    /**
     * @param ListProductServiceInterface $service
     */
    public function __construct(ListProductServiceInterface $service, Container $container)
    {
        $this->service = $service;
        $this->container = $container;
    }

    /**
     * @inheritdoc
     */
    public function get($number, Struct\ProductContextInterface $context)
    {
        $products = $this->getList([$number], $context);
        return array_shift($products);
    }

    /**
     * @inheritdoc
     */
    public function getList(array $numbers, Struct\ProductContextInterface $context)
    {
        $products = $this->service->getList($numbers, $context);
       
        foreach ($products as $key => $product) {

            /** @var Struct\Attribute $attr */
            $attr = new Struct\Attribute([]);

            # article id of canonical target
            if ($product->getAttribute('core')->exists('attr2')) {

                $id = Shopware()->Modules()->Articles()->sGetArticleIdByOrderNumber($product->getAttribute('core')->get('attr2'));
                if ($id) {
                    $attr->set('canonicalTarget', (string)$id);
                }

            }

            $product->addAttribute('custom', $attr);
            
            # retrieve it in your tpl with {sArticle=$sArticle.attributes.custom->get('canonicalTarget')}
        }

        return $products;
    }
    
 }