Define "append". xlswrite will add cells to an existing workbook. If the cells are written to a location where there are no existing cells, then the existing cells will remain intact. If you write to cells in a range that overlaps existing cells, the cells in the overlap region will get replaced and the non-overlapping ones will remain.
If you want to find out the cell range that is used by your workbook, you need to use ActiveX and ask for Excel.worksheets.Item(sheetIndex).UsedRange or Excel.ActiveSheet.UsedRange.