Hi,
You can do it like that:
$orderProductClass = hikashop_get('class.order_product');
$orderProduct = new stdClass();
$orderProduct->order_id = $order_id;
$orderProduct->order_product_name = 'My product';
$orderProduct->order_product_price = 10;
$orderProduct->order_product_quantity = 1;
$orderProductClass->update($orderProduct);
Then yes, you need to recalculate the total in $order->order_full_price (basically, you just add the amount to the value you already have in order_full_price in the database for that order).