The following describes my understanding of the process. I have done this for KORD and it works.
You can modify an airport ICAO.dat using the X-Plane World Editor, available here:
https://developer.x-plane.com/tools/worldeditor/. Your updated airport can then be submitted to the X-Plane Scenery Gateway (if you have X-Plane or if you can find someone with X-Plane and is willing to upload it to the Gateway). On the next FG scenery build, the airport should be included in FlightGear.
If you don't want to wait that long to view this new airport layout in FG, you will need to generate it using TerraGear (see
https://wiki.flightgear.org/TerraGear).
You will also most likely need to change the NavData files for the new runway and possibly files in the "data/scenery/airports/I/C/A/O" directory: ICAO.ils.xml and ICAO.threshold.xml.
A new NavData directory will need to be created with entries in the subdirectories /apt, /awy, /fix, /nav, and /poi. For files in /apt and /poi, you can get away with copying the new ICAO.dat into these subdirectories. For the others, you must search the forum for the format, then create new files as appropriate.
All these files will be put in a custom scenery folder for your personal use now, as they can't be added to FG easily and the process takes a long time (see
https://forum.flightgear.org/viewtopic.php?f=5&t=30734 and
https://forum.flightgear.org/viewtopic.php?t=34168&p=338373#p338610.
So it is quite an involved process, but not terribly hard to accomplish. Perhaps, though, you can at least have things ready for the next World Scenery build?
I hope this information is helpful to you.