Installing the XBee Shield & Motor Shield onto the same Arduino board is physically impossible
Not necessarily. Depends on whether you have the fully assembled motor shield, or a kit, and which XBee shield you are referring to.
If you get the motor shield as a kit, you can change the header pins for stacking header pins. Then, you could mount another shield on top of it that does not use the same pins.
There are different XBee shields, too. Some use the ICSP pins. Others do not. Again, if you get an XBee shield that does not use the ICSP pins, and you get it in kit form, you can assemble it using stacking headers.
can I do it just by installing the Motor Shield and wiring the necessary pins from Arduino to XBee?
As far as I know the XBee Shield just needs the TX/RX pins and the ICSP.
The ICSP connection is supplying power and ground only. You can use +V and Gnd from other places instead.
Which pins does the Motor shield need?
Depends on what you are driving with it.
See the question on pins.