Interfacing 9-Axes Motion Shield with RPi and WiFi Shield
Q1. The official Arduino page says that the shield doesn't have any external power source and can only be powered up by stacking on top of an Arduino board. How can I use this shield with a Raspberry Pi?


By providing the right power to the right pins.


  Q2. A page here shows that it has support for the Ethernet Shield but doesn't mention WiFi shield anywhere. Does this mean that I cannot use my WiFi shield with it? I plan to stack the 9-axes shield on top of the WiFi shield which in turn is stacked over Arduino Uno.


No, it says it is compatible with the Arduino Ethernet which is not a shield, it is an Arduino.

What other shields you can use it with depends on what pins those other shields use.  Look at the schematics. Read the manuals. Do some leg-work.