Adding SPU MPU9250 to PX4 v2, Arduplane 3.7

I have a PX4-v2 based ardupilot and am running arduplane 3.7 with some custom code/flight modes etc. I do not want to switch to 3.8 or higher because it would likely break the aforementioned custom code due to the significant config changes between 3.7 and 3.8.
How can I add support for the MPU9250 IMU/compass over SPI?

Thank you.

Asking on the ArduPilot forum is probably better than here, in your case (