My approach if I were actually going to void any waranty, and just wanted to see if it was possible would be this:

First, check the value of the potentiometer, and see what voltage it outputs throughout its range with either a Voltmeter or Oscilloscope.

More than likely, its output will be from zero to five volts.

You can then try to replace the potentiometer with one having a larger or smaller value that will give you the addtional rotational range you want, but still output the same voltage as the intial one at its limits.

Let us know if you can get it to work.