The ADC128S102EVM pin description thread had an answer that stated: "If you have VA set to 5V, you can use IN4 and IN5 with inputs up to 5V. IN0 - IN3 and IN6 are limited to 3.3V because of the op amp that is in the signal path, which is powered by a 3.3V supply. Putting 5V on IN0 - IN3 and IN6 will damage the op amp. If you want to use these inputs with a 5V input signal, you will need to modify the board."
Can you source 5V into the "3.3V rail", such that all devices receive 5V power? It looks like all of the components can handle 5V. You would just need to make sure VA is jumpered and receives 5V via VA-EXT. Are there other modifications that would be needed to allow you to input 5V signals into the ADC?