When you use an Excel data source in Word 2008, Word uses a converter to
get the data from Excel. Also AFAIK, Word simply does not know how to
write data back to the Excel workbook, which is presumably why it does
not let you. You can alter the workbook content directly in Word, but
Word will not notice until you either close and reopen your mail merge
main document, or go through the data source selection process again.
In simple cases where you have no formulas in your sheet and no
multiline data etc., a viable workflow might be:
a. export the data from your worksheet to .csv format
b. use the .csv file as your data source. You should be able to edit
its contents using the relevant button in Mail Merge Manager.
c. when you're done, re-import the .csv data.
But personally, I'd rather stick to editing the data in Excel.
Peter Jamieson
http://tips.pjmsn.me.uk