Mergefield in Word 2013 - change format of the zip code

I have a mergefield in Word 2013 for the zip code and when I merge my document the zip code is not formatted properly for the zip codes that contain nine digit numbers - the dash is missing.  Is it possible for me to edit the mail merge field to include the dash if the number is nine digits?  I was able to edit the merge field for the date, i.e.,   I added the following formatting data: \@"dd MMMM yyyy" into the merge file of date. And the merge field of date showed the full date written out March 27, 2017 in lieu of 3/27/2017 -- trying to determine if I can do the same with the zip code field.  Thanks,