Hi Leo,
I cant really help much but certain things are clear...
In order to use an analogue signal it will have to be converted to digital before Mach can use it.
On possibility is to use two ADC's (or one two channel) and communicate those values via Modbus.
The ADC's commonly used in microcontrollers are 12 and 14 bits and may therefore lack the resolution
you require. All in all this approach could be a major project.
I may be mistaken but I think HICON have an addon board which accepts two channels analogue. There may well
be others but I recall seeing that feature listed when I was surfing around.
Craig