I assume you mean that you want your ZIP drive to be used as the default location by Word?
You can do this, but you'll have to remember that the path must always be the same. In other words, if you create a folder on the ZIP disk called 'Mywork'and set it as the default saving location in Word you must remember to create an identical folder each time you insert a new disk in the ZIP drive.
To change the saving location, go to tools and select options. Then select the tab marked 'File locations'.
Click once on 'documents' to highlight it, and then click on the 'modify' button. Browse to your ZIP drive and select the folder location you want to use.