Editing a response

We have a "hidden" question in our form that we wish to have "office use only" by assigning a submission number. As the facility to automate numbers is not yet available we chose to have this as hidden question which we would manually complete using task: edit responses. It does allow us to enter a number BUT will not let us save unless all empty fields are completed. We only wish to edit one question not the whole response and not all questions are mandatory. How can we do this?