If you are familiar with VBA and RW, then you can customize that report by
adding a calculated field which can pull and display it from the invoice
table the necessary fields.
Please have a look into the david's link.
http://blogs.msdn.com/b/developingfordynamicsgp/archive/2008/07/17/vba-accessing-any-sql-data-from-a-report-example.aspx
Hope it helps.
Arun.
arunprasathvm@gmail.com