wow... I am not so sure I am following that much..... Here is what I got correct me if I am wrong.
You used to have in the system two wires. 1-Ground, the other one change resistance according to what switch was pressed..... right? The old chip used to get this signal and do somestuff accordingly.
Now you have 3 wires. 1-Ground, 1- Signal (that changes resistance as before) and 1- I will call reference......
It seems like Exo mention that the chip is doing a ADC convertion. What this does is checks what voltage you have at certain pin and it assigns a digital value to that voltage according to the resolution of the ADC. So a 8-bit ADC can only generate 255 numbers for voltage change.
The switches as I can understand are connecting or disconnectig resistances to change this voltage.... by what is call a Voltage divider. I am not sure if you are familiar with potenciometers, but they work sort of that way.
Analog to Digital Convertors need 3 wires most of the time. 1- Would be your ground, 1- a signal (analog voltage-changing voltage), 1 you reference voltage. That sound awfully close to what you have now. Some processors have and ADC built it.... and they use their voltage supply as a reference, therefoe you don't need an external reference pin. But some can be configure as to have different voltages as a reference in case you want something different from your chip voltage supply.
I am guessing that if you connect the one voltage that goes to the switches to this new pin that will set the reference voltage as before. And just have the signal pin fed into the signal pin and the fround to ground.
I am assuming a whole bunch of stuff.... if you have schematic of the switch arrangement and chips, that might clarify a whole bunch.
Good Luck
Ivancho