Korg Poly800/EX800 Users

I recently saw someone was selling a Poly-800, which he had modified.
He had the standard filter pot mods, but he had also installed a wheel
which you could change the parameters with (instead of the value
up/down buttons). 

I was to ask him how this was done, but i forgot about it, so I wonder
if anyone here has done the same mod and would like to explain how to
do it here, because it'd be awesome to control everything faster with
a wheel!