There are basically two ways to accomplish that:
Collect data from sources and feed it to the dump1090 instance with the net-only option. Here, you need to ensure that this combined data is not fed anywhere(flightaware and others).
Or you do a hack of the web interface, where you create a combined aircraft.JSON with data from all your sources. I did something similar, but added data from the opensky API to my dump1090 web interface.