MorganS:
That's going to be tough. It may be easier to rip out the display from the commercial unit and put your own display in.
I believe OP wants to pretend to be pH probe, rather than trying to read the probe.
The voltages are not the problem, they're big enough: 60 mV per pH point. The problem is with the tiny tiny current, which is 1 nA at most and more likely in the order of 100 pA.
The other day I actually built a pH sensor circuit from scratch on protoboard, and it works quite well - better than expected, as in it actually just worked - considering the noise it just has to be picking up. Offset voltage to the shiel, OpAmp amplifier with 2x gain (going to change that to 3x to cover more of the 3.3V range - otherwise that 10-bit ADC is really too coarse), and then to the analog in of a NodeMCU board.