Hello,
I would like to use the Xbee S1 board to read an analog voltage (so using the ADC) and send it out to a board (STM32) through the USART interface. All would be done using one Xbee module
It is possible?
Yes it is. See http://knowledge.digi.com/articles/Knowledge_Base_Article/Analog-to-Digital-Conversion-on-the-XBee-802-15-4/?q=xbee+ADC&l=en_US&fs=Search&pn=1 as well as the ADC section of the product manual.