Comment 4 for bug 123955

This still occurs for me in ubuntu 7.10. If you try to add an item to the system menu, it simply doesn't work. No error message or crash or anything. However if you look at the help page for alacarte it says this:

You can modify the contents of the following menus:
              Applications menu
              System ▸ Preferences submenu
              System ▸ Administration submenu

So this behaviour is probably by design or omission.