The XBee modules can do simple digital or analog I/O, but both the DHT22 and BMP085 sensors require a microprocessor for communication. You could add an Arduino (or a Boarduino
for a smaller, less expensive package) to the outdoor transmitter. You would connect the sensors to the arduino/boarduino (see the sensor tutorials for details) then connect that to the XBee adapter via serial.
A 6xAA battery pack
with either alkaline or rechargeable NiMH cells should power this for a good long time.