Built in support would be messy because the torque and power curves are a simple mathematical model that allows a reasonable estimate of their shape using the peak torque and peak power rpm and values. It's not perfect as it is. Adding a mathematical model for restriction would deviate quite a lot from LFS' results probably. Your best bet is to take the current vehicle and edit the engine specs. You just need to enter the new peak torque and peak power, and their rpm as given by LFS.