First you need to find out which key mapping you are using. Usually it will be US, it might also be en_US, ca or else. Locate the file, usually in /usr/X11/lib/X11/xkb and edit it with your favorite editor. For me the file is called /usr/X11/lib/X11/xkb/symbols/ca.
The file lists all the key codes and what they do. The key codes for the Win95 special keys are LWIN, RWIN and MENU. All you need to do is add them to the list, with the functions for them. I decided to map the left
WIN key to "@" and the right WIN key and MENU keys to "{" and "}". Here are the lines I added:
key
key
key