According to the datasheet (page 17 - "DC Characteristics of the XBee/XBee‐PRO") the ADC input voltage range on a XBee series 2 is typical 1.2V and max. VREFI (internal reference voltage).
There is a note on the "DC Characteristics of the XBee/XBee‐PRO" table referring to the ADC input also:
"Note – The signal-ended ADC measurements are limited in their range and only guaranteed for accuracy in
the range 0 to VREFI. The nature of the ADC’s internal design allows for measurements outside of this range
(+/- 200mV), but the accuracy of such measurements are not guaranteed."
I can't find any maximum numbers for the ADC inputs in the datasheet though.
A simple way to step down the original voltage (in this case 3.2V) might be a simple voltage divider consisting of two resistors. If you look for very fast signals you might consider specialized level shifter ICs.