It is probably easiest to just do this on the completed page. Run the wizard with one of the defaults and then open up the database and add the columns you want.
Then open the confirm.php page that is created and double click on the store cart summary in database server behavior and store cart details in database server behavior to set the values of the added columns.
You will have to do the same on the pp_confirm.php page if you are using paypal express checkout.