If you've got a device that listens to serial over USB, just hook it up to a USB port on the Pi. Theoretically, it´ll show up as a /dev/ttyUSB0-9 port (pick a number out of a hat). If it's USB, you won need level shifters. For the Xbee, does that use RS232 or UART? UART 3.3V is fine, RS232 is not.