Absence balance adjustment free text box

Summary:

Content (required): HR have asked if it's possible to have a free text box where they may record details about absence balance adjustments carried out (in more detail than would be possible by simply extending the balance adjustment reasons list, as they want to record calculations performed etc.). Is there any way to add a text box, either to the Adjust Balance area, or somewhere on the main 'Manage Absences and Entitlements' page?

I thought perhaps a new DFF would achieve this, but I'm not sure if it's even possible to have a free text field DFF? If it is, any advice on how to go about adding this either against the balance adjustment itself or anywhere on the main page would be much appreciated.