You can do this pretty well using a macro that handles merge events in
the mail merge template. I have a mail merge that has a word table in
each letter that may contain more than one row. I just handle the
MailMergeAfterRecordMerge event, and check my datasource for additional
rows that may need to be inserted into the table. It's in VBA so it's a
little clunky, but it works.

I gave up on MailMergeBeforeRecordMerge Cancel option as it always
seems to cancel the entire merge.

_Randal