The shoestring site has the instructions on how to add a guide port to a controller that doesn't have one. It involves soldering wires onto the hand controller board - the other end of the wires has a RJ12 plug on it. You then need a GPUSB to connect the RJ12 to the computer via a USB port. Here's the link
http://www.store.shoestringastronomy.com/eq_mod.pdf