Dear Vincent,
StellarIP creates ISE firmwares.
Reference software as FMC30RFApp are communicating with the firmware over Ethernet.
So if you modify a star, ie adding registers you will need to modify the star in StellarIP but also the reference software in order to use these registers, but I guess this makes sense.
I think you should go through the 4FM Getting Started Guide, especialy chapters 4.9, 5, 6. If after that you are still confused, then forget about StellarIP, as soon you got your ISE project, you can focus on ISE. Then you modify the stars as you want, normal ISE/RTL job. You will then need to write C++ code to support your modifications.
Best Regards,
Arnaud