Adding and MPG to an existing control.

I think this isn't exactly on topic for this list but I thought this
would be the best place to ask. I want to add a Manual Pulse Generator
to an existing commercial CNC control and was looking for anyone that
could offer advice or at least point me to a place I can find useful
information. The control is a Siemens 810A (Sinumerk) that is on an
old Emco lathe I have. There are versions of this control that have
MPGs and I heard there was a kit once available for somewhere around
$900. Ideally I'd like a wheel for the X axis and a wheel for the Z
axis, but at this point I'd be happy with one wheel that is switched
between the 2. Can anyone point me to a schematic for a pulse wheel
pendant that might work? And does anyone know this control enough to
know what I may need to do to integrate an MPG to it and what might
need to be enabled in the software for it to work properly?

Thanks-