There is actually an arduino shield that let you integrate arduino with the Lego microcontroller, so you can expand the number of inputs and outputs, though this is mostly a transition from lego to arduino, because at that point, you better off doing everything through an arduino (except that you get to use your lego motors and sensors with it this way). Here is a book that teaches about how to do it and talks more about the arduino shield:

http://shop.oreilly.com/product/0636920024316.do