Using Arduino standard SPI library in Ardupilot source code

I am hoping to use Arduino specific functions in the UserCode.cpp file of ArduCopter. I specifically want to use the functions in the SPI library. Also, I am using the Pixhawk flight controller.

Are these already included in the ArduCopter source code? Or do I need to insert them into header files and #include them to use them?

ArduPilot, despite the name, has nothing to do with Arduino.
It has it’s own Hardware Abstraction Layer and an underlying RTOS (Nuttx or ChibiOS)
You must use hardware related functions provided by the above.
Check an existing driver code for ideas, and use latest version of code with ChibiOS RTOS.