You can reopen the work order by changing the status at WO-A and then saving it and you can then add the labor but that will not change the actual item cost.  If the parts are still in stock, you could back out the Finished Production and re-process it after the labor has been added but if they are already shipped, you can get the Job Cost correct but the extra labor cost will go to WIP Variance when you re-close the WO.