Hi Randy,
To catch this utility, the ExcelMarkerMouseController from the ExcelSelectionMarker sample can be used which, enables dragging at the bottom right corner of a cell and on release you can get the affected cell range. At this point you have to affect copying into the range using whatever algorithm you want to use to fill the expanded range. Below is a forum thread that talk about the similar event.
http://www.syncfusion.com/support/Forums/message.aspx?&MessageID=23046Best regards.
Haneef