Add custom MPG to UC100 / Stepcraft D.840

Hi,

I'm up and running with my D.840 and UCCNC (via a UC100 controller). UCCNC can accept an MPG wheel using two pins. I have a selection of rotary encoders, which I'd like to use one as an MPG. I have two questions;

1) Is the UCCNC software input on two pins designed for generic rotary encoders?

2) The stepcraft and UC100 manuals both say pins 18-24 are ground pins. The stepcraft manual says pin 13 is the 4th axis reference and 15 is N/A, both of which are input pins on the UC100. As I don't have a 4th axis, could i use pins 13 and 15 and any of the ground pins to connect up a rotary encoder? I'd use a breakout box for the cables. Could I just tap (solder directly) into these pins, or would I need to break the connection to the Stepcraft board?

Any answers or help will be gratefully received!

Dave.