ADC has an input for an analogue signal and gives a digital output (as parallel or serial bits) or loads a byte or two if in MCU. So I couldn't get what you meant exactly by "representing ADC as a simple load".
Thevinizing is meant for linear circuits. So you cannot accommodate the functionality of the ADC circuit since it is non-linear.
However, you could Thevenize the input of the ADC. It would simply be the input impedance of the ADC.
You could also Thevenize the outputs for given states if they are linear. They would represent the logic voltage level the output can achieve with no load plus the internal resistance of the output.