Xmega ADC ground as internal input
To my knowledge, it is not possible.



I have used the 16/32A4, 64B3, 128A3 and 16D4. None of them have this capability. I always dedicate a separate ADC NEG pin for GND and measure differentially against it. But it also lets you measure the ground offset.

In case you don't have any spara pins and/or you already manufactured the board, there's probably no easy way for you...