Hey, that works great!
Funny that I've been using Window Maker for almost two years but I never thought of that myself. I just took it for granted that Debian menu is incompatible with the menu editing tools that are available in WMaker. Thanks basileus for an excellent tip that helped me to get rid of one of the very few gripes that I have against Window Maker.
In case you don't have the default Window Maker menu, "plmenu", under ~/GNUstep/Library/WindowMaker/ , you copy it from /usr/share/WindowMaker/ . In order to add the Debian menu to Window Maker's default menu, add "External Submenu" in WPrefs or wmakerconf and point it to "/etc/X11/WindowMaker/menu.hook" (that is updated every time you add or remove programs).