Hello John,
I apologize that this question was somehow missed.
I will answer this for the purpose of anyone else who might have the same question.
The ADC is a current input ADC. So yes you could use it as a summing node but the 2K internal series resistor will limit the amount of signals you can sum before noise becomes an issue. The impedance of the external summing resistors will need to be greater than 18k to have an efficient signal transfer into the ADC. So this means that the resistors will become large very quickly and noise performance will suffer.
So for this reason it is probably best to drive the ADC with a high quality low noise opamp and have an earlier stage perform current summing with the traditional virtual earth configuration. This should yield the best noise performance.
Thanks,
Dave T.