XBee ADC without transmitting
I have not seen a function that can do that in the standard Xbee firmware. I'm sure it could be done if you could develop and download custom firmware to the Xbee, but that's not something that is open to hobbyists AFAICT.

That being said, a small MCU with an ADC and a serial port of some sort will cost you maybe a dollar or two, and needs just the supply voltage and a decoupling capacitor for support. The Atmel ATTiny85 comes to mind.