The only way to enable usbNetwork is to use the hacks in the thread I linked to.
Amazon removed the usbNetwork support files from the regular Kindle 3. There is the very remote possibility that they left them on your Kindle as somebody on this forum received a Kindle with the test/debugging tools still loaded on there but I've never seen anybody else mention that they've got them. It is easy enough to test as the usbNetwork command will work if you do have the tools or have the hacks installed. If you don't then it will do nothing and harm nothing.