You can hack it in place in the simulator module. https://github.com/PX4/PX4-Autopilot/blob/f571b29ae8152cc0b3d0f6bb7f160e3a44a8d9ed/src/modules/simulator/simulator_mavlink.cpp#L324-L341
If there are specific ways you want to spoof GPS or simulate failures we can extend it as needed.