Currently, there is no way to add a comment to individual feedback activity responses.
First, some context: In a blended face-to-face and on-demand course with many activities, I have created a feedback activity titled "Deadline Extension Request" with three free-response questions: 1) What is the name of the activity? 2) Why did't you complete this activity on time? and 3) Why should I allow you to complete this activity late? Students who miss an activity and wish to complete it late utilize this feedback form to log official requests. It has proven very useful except for the fact that I cannot, within this activity, keep track of my follow-up actions. I could of course delete individual responses after following up with the student, but that would also delete any record of the student's request as well.
Ideally, I would like for the "Show responses" report in a Feedback activity to include a column for adding comments, much like the "Submission comments" in an Assignment activity.
I apologize if this isn't the right venue for posting such a request.