Someone may have a better way but this worked for me. But please try it on a
copy of the worksheet in case I am not understanding you correctly. Say my
first zip code is in cell A1. In cell B1, enter the formula
=A1&","
and copy it down. This causes column B to contain the zip code with the
comma after it. Then I copied Column B and used Edit, Paste Special, Values
to paste it over Column A.
Rather than changing the data, if these are all 5-digit zip codes (i.e. all numeric, no 9 digit
with a dash -- which makes them text), you can use a custom number format: #\, or 00000\,