I think you would have to do this dynamically on the dropped list. Here is a sample that uses CurrentCellShowingDropDown to initially populate the list with the 5 items items, and to subscribe to the ListBox.SelectedIndexCanhed event for the dropped list. Then in the SelectedIndexChanged event handler, it adds the 2 items when you hit the last item in the original list. It also uses CurrentCellCloseDropDown to unsubscribe to the SelectedIndexChanged event.
ColNameSample_5960.zip