One way to do it is before doing the datapump task to load data to the workbook, set up an ActiveScript task.

Then, do some VBScript to

open Excel

open the workbook

select the right worksheet

select the ActiveRange

delete the ActiveRange.

Save the workbook

close Excel

Sorry I don't have the code off the top of my head right now, but you should be able to find some VBScript examples to do the bulk of it. Just google for it w/o specifying DTS, maybe something like: "VBScript Excel"