Probably what you are really asking is if you can use a usb to serial cable to connect AdrupilotMega to your computer directly instead of XBee? I am sure you can, using FTDI cable or similar device. I don't think that XBee API protocol and addressing is used. Any experts on this?
Yes, you can do that by changing
#define GCS_PORT 0 in APM_Config.h
but its actually suggested to use the telemetry port, even if you dont have xbees.
If you have a FTDI cable, make up a pigtail with the tx and rx (and optionally ground) connected to the telemetry port.
Very similar to a GPS to FTDI cable, but without the power.
Yes, the solution appeared itself. My Xbee modules were delivered so quickly, so I had no time to test via usb without them.
Nevertheless, thank you, now I know that there is the other way of connection.