Note that the DDT object only allows reading. The assumption is that you are using the Excel sheet to read through data records to be used in processing a data driven test. Any output to be written would need to be written to a separate copy of the file using Sys.OleObject('Excel.Application') as per the link @cunderw posted.
Why automate? I do automated testing because there's only so much a human being can do and remain healthy. Sleep is a requirement. So, while people sleep, automation that I create does what I've described above in order to make sure that nothing gets past the final defense of the testing group. I love good food, good books, good friends, and good fun.