You can't do this with Word's standard mailmerge tools. However, you can do so using the approach detailed in my Microsoft Word Catalogue/Directory Mailmerge Tutorial, available at:
http://windowssecrets.com/forums/sho...ngs-(2000-2010)
or
http://www.gmayor.com/Zips/Catalogue%20Mailmerge.zip
The tutorial covers everything from list creation to the insertion & calculation of values in multi-record tables in letters. Do read the tutorial before trying to use the mailmerge document included with it.