Hi,
As Jerome said, the information is
encapsulated but the information will be there
in the report.
If you want it as its own column, then create a custom field of the table "order" via the menu Display>Custom fields and replace the line:
$order->order_payment_params->purchase_order = @$_SESSION['hikashop_purchase_order_number'];
by:
$order->XXX = @$_SESSION['hikashop_purchase_order_number'];
where XXX is the column name of your custom field.
You'll then get your value is a separate column with its title being the column name of your custom field.