Можно добавить товар, примерно так
require_once('PSWebserviceLibrary.php');
$webService = new PrestaShopWebservice(PS_SHOP_PATH, PS_WS_AUTH_KEY, DEBUG);
$xml_product = $webService->get(array('resource' => 'products?schema=synopsis'));
$resources_product = $xml_product->children()->children();
$resources_product->quantity = 0;
$resources_product->price = 1;
$resources_product->out_of_stock = 2;
$resources_product->name->language[0][0] = 'test';
$resources_product->link_rewrite->language[0][0] = 'test';
$resources_product->associations->categories->addChild('category')->addChild('id', 1);
$xml_product = $webService->add(array('resource' => 'products', 'postXml' => $xml_product->asXML()));
......