Hi,
This should be fine. However, I would recommend to only add the attributes you want in the $order object, like this:
$orderClass = hikashop_get('class.order');
$order = new stdClass();
$order->order_id = $order_id;
$order->custom_auth_number = $authNumber;
$order->custom_txn_id = $transactionId;
$orderClass->save($order);
Note that you first need to have the columns custom_auth_number and custom_txn_id in the hikashop_order table.
If you don't, the MySQL query will crash.
So you first want to do an ALTER on that table to add these columns if you don't already have them.