Debra Dalgleish posted this for a different question:

In the Mail Merge, after you select your Excel file as a data source,
you should see a 'Confirm Data Source' dialog box.
  (If you don't see the dialog box, change the setting in Word --
   under Tools>Options, General -- add a check mark to
    'Confirm Conversion at Open')

From that list, choose 'MS Excel Worksheets via DDE (*.xls)', and your
formatting will be retained.

If you have to connect through a different source, you can format the
fields in the Word document. For example, to specify a number of decimals:

1. In Word, in the Main Document, press Alt+F9 to view the field codes.
2. Find the field code for the number. It will look something like:
           { MERGEFIELD FieldName }
3. Add a switch, to format the number with two decimal places.
          For example:
           { MERGEFIELD FieldName  \# "#,##0.00" }
4. Press Alt+F9 to hide the field codes.
5. Save the Main Document

(I bet you could modify it for your situation.)