This may helphttp://arduino.cc/en/Tutorial/SoftwareSerialExample
he Arduino Due is a microcontroller board based on the Atmel SAM3X8E ARM Cortex-M3 CPU (datasheet). It is the first Arduino board based on a 32-bit ARM core microcontroller. It has 54 digital input/output pins (of which 12 can be used as PWM outputs), 12 analog inputs, 4 UARTs (hardware serial ports), a 84 MHz clock, an USB OTG capable connection, 2 DAC (digital to analog), 2 TWI, a power jack, an SPI header, a JTAG header, a reset button and an erase button.
Have you tried using one of your extra serial ports to do something like this ?
// Open serial communications and wait for port to open:
void loop() // run over and over
I've never tried something like that so the syntax could be wrong but you get the idea. Read in from one port (the xbee) and write to another port (Serial Monitor). If that works then the next step is parsing the incoming code before sending the parsed data out. Persue that angle.