There's no practical way to do this. This thing is not meant to replace your computer. USB hubs are easily thicker than the Kindle itself. Unless you want to your Kindle to be half-opened, I don't think it would be possible to add a USB hub.
The only possible way to do add an external keyboard without making your Kindle look like a malformed router would be to add a bluetooth adapter (
http://www.griffin.net/2010/01/hacki...dx-part-1.html). Then, supposing you could write your own driver, you could hook up a Bluetooth Keyboard and use that... (but I doubt that's possible unless you are an experienced linux programmer) The end effect would be something like the iPad with an external keyboard.