Asda. It's not possible to edit the default scenery. It is possible to modify it by overlaying areas using a conventional scenery design program. I already covered this in your topic in the FS2002 forum.
Very basically: It's possible to visually update an existing airport with a scenery design application like Airport for Windows or FS Scenery Creator. You will find more details on these in the Scenery & Panel Design forum.
Once this is done the virtual scenery used by AI traffic (new runways, taxyways, parking ramps, gates etc) can be updated with AFCAD.
My first suggestion would be to try the design programs & see which one suits you. Both come with instructions & tutes.
Airport for Windows.
http://www.airportforwindows.com/FS Scenery Creator.
http://fssc.avsim.net/downloads.html