Use Custom GPS Data for on-board computations

Assume that there is no reliable GPS data available at some location. Let’s also assume that I somehow know the GPS data at every point in space magically, but the sensor is not able to find it. Is there any way through which this external custom GPS data can be sent to the onboard computer (via rostopic, telemetry etc.,)? The onboard extended kalman filter can still make use of this custom GPS data, to generate stable control signals?

It would be really grateful if I could get some suggestions / tips etc regarding this.

Thank you.