Order.php

<?php

// path: /override/classes/order/Order.php

class Order extends OrderCore
{
    public function __construct($id = null, $id_lang = null)
    {
        $this->webserviceParameters['associations']['order_rows']['fields']['additional_content'] = ['required' => false, 'xlink_resource' => 'customizations'];
        parent::__construct($id, $id_lang);
    }

    public function getWsOrderRows()
    {
        $result = parent::getWsOrderRows();

        foreach ($result as &$item) {
            if ((int) $item['id_customization']) {
                $item['additional_content'] = $this->getCustomizationSummary((int) $item['id_customization']);
            }
        }

        return $result;
    }

    public function getCustomizationSummary($id_customization)
    {
        return "<b>Test {$id_customization}</b>"
    }
}