If your Expansion Hub was purchased BEFORE December 2021, it already has an internal IMU installed and you do not need to follow these steps.
There are a few options that will work for giving your Expansion Hub Gyro/IMU function.
Integrating Gyro with our Logic Level Converter and Sensor Cable Adapter - This is directly supported in the FTC Programing environment but is just a single-axis gyro, not a full IMU.
navX2 Sensor Bundle - Also supported in the FTC programming environment. Code examples are listed on AndyMark's page, and this product includes the correct cables to use within FTC.
Adafruit 9-DOF Absolute Orientation IMU - This is the same IMU as in the Control Hub, but will require you to either create an adapter cable or solder a cut sensor cable to the board. Plugging this in and configuring the IMU on I2C port zero will allow you to use and program the same as an internal IMU.