I have done this and it's quite easy (although it can be a little time consuming).
Pull the radio out of the dash and disconnect it to get it out of the way. Locate the original plug (the satnav one). There will be a shroud around the plug that is removed by sliding it to one side. It may have a catch somewhere on the shroud, but it is pretty easy to figure out (don't loose the shroud!). Remove your aux input from the radio plug and remove the shroud on that connector as well. Compare the plugs and make a note of the orientation of the plug itself and the wire location - pin 1 is marked on the naked plug by a small arrow or triangle. Now you will need to take a very small flat-bladed jewler's screwdriver (a sewing needle works very well as well) and gently pry the little tabs that hold the pins in place on the aux input plug and pull the wire out of the plug. Now you will need to put that pin into the satnav plug in the same location as it was in the aux plug (if you can, do only one pin at a time so they don't get mixed up). Now do the other two pins the same way. Once you are satisfied that you have the pins in fully, place the shroud back over the plug and insert it back into the radio connector. Reinstall the radio and check for operation before you button everything back up.
I am *VERY* interested in the spare plug and shroud you will have after you make the modifications. I have seemed to misplaced mine and I need the plug for the new car (it doesn't have satnav). Please PM me if you would like to sell it to me.
For reference, I have a diagram of the plug on my website
HERE. It's connector CB891-C, pins 3, 4 and 10 (I'm pretty sure anyway). Best to make a note of the pin number of the aux connector and the wire color before you move it to the satnav plug.