If you are operating in a full-fledged, commercial flight simulator environment, then first you have to solve the problem of obtaining the requisite position, velocity and time (PVT) data from the simulator. That promises to be an even bigger problem. Such systems are not exactly open source. Do you have access to that data already?
If you look at the marketplace for position spoofing in both iOS and Android environments, you'll find that pretty much all solutions are associated with achieving only a fake static position, which alone is sufficient to deter tracking or to breach location-based restrictions on access to information and services. Thus there doesn't appear to be anything in the marketplace to help you.
The "A" answer would be to utilize a commercial GNSS simulator (
example) that broadcasts an actual signal inside the commercial simulator facility, which would then be picked up naturally by the GNSS receivers in iOS and Android products. The GNSS simulator would need to be interfaced to the flight simulation software, which again brings us back to Problem Number 1. And you'd need to make certain that the simulator and building structure, and GNSS power levels, are such that the simulated signal does not escape from the facility lest you interfere with the real world.
I suspect that if you contact the maker of the flight simulator you might find that they already have a relationship with a manufacturer of GNSS simulation products.