Can the RTK EXPRESS be modified or upgraded to have IMU and L band function?
You could use this https://www.sparkfun.com/products/19390 and a choice of IMU https://www.sparkfun.com/categories/160 to add in those features 
If using qwiic for both make sure you disable all but one set of pull up resistors
In short, the RTK Express is not upgradable to L-Band. TS-Russell is correct, you could add the NEO-D9S breakout over Qwiic but I’m not sure if the firmware on the RTK Express would identify it correctly. Additionally, adding an IMU would be a very large change to the system that is not possible at this time.