Is this an online form? Just add the missing field on the Design Tab. This will ask you to close the form first. Do so and right after adding your field re-open the form. You can Open the form in the Distribute Tab.
This will add the column you want in the response table and will also add the field to the fillable form for your future fillers. You can fill by hand the data rows that have already been submitted in the response table. But all new submissions should have that value filled out by the filler.
hope this helps
BTW, if this is a PDF form then you will need to Download the submission enable PDF again and redistribute the PDF form with the new field added.
It appears that I was able to do the extra field and sort them. Thank God for small favors!