The maximum sample rate that can be achieved while using one A/D line is 1 sample/ms or 1 KHz (Note that the modem will not be able to keep up with transmission when IR & IT are equal to “1” and that configuring the modem to sample at rates greater than once every 20ms is not recommended).
If I read that properly, that implies 50 Hz maximum recommended sample rate. Will that be enough for your purposes? Or are you having something else do the A/D conversion?
Skimming through the manual, it appears that you would treat the Xbee as a serial device: construct a serial object with serial() and fopen() it; send it some AT commands to configure it for A/D use, and then read the frames of data according to the section "I/O Data Format" on page 13 of the above manual.