Can an ArduPilot support an additional shield kit, (aka "shield stacking"), such as the AdaFruit Motor Shield? This would be additional to the ArduPilot Shield Kit shown here:
No it doesn't. But the next version, ArduPilot Mega, will. Different shields have different ways of interfacing with the main board and other shields, depending on what makes the most sense: serial, I2C, high/low pins, SPI, etc...