In the callback for the ring, you could check if the label is "Add an entry". If it is, you could install a popup panel with controls for a label and its value, and OK and Cancel buttons. If OK is clicked, InsertListItem using the new label and value, then RemovePopup(0). If you insert it at the current index, "Add an entry" will get pushed down. If Cancel is clicked, just RemovePopup().
See the attached example in CVI 6.0.