Hi Eric.

I think the wzzip switch you want is -u.  It would work something like this:

WZZIP -P -R "d:\archives\my files.zip" -u "c:\my files\*.*"

The first time you run such a line, it adds whatever files and subfolders 
you might have stored in a folder called "c:\my files" to an archive that is 
located in "d:\archives\my files.zip"

The next time you run the same line, it updates the archive with whatever 
files/folders you've added to "c:\my files" or any files you changed in the 
meantime within that folder.

You could run the command manually from the command line, or put it into a 
batch file which would do it for you.

If you want to see a list of supported switches, install the wzzip addon, 
then type wzzip /?   After that, click the hyperlink that says "WZZIP 
reference".