The airports are already "dual purpose". The game engine makes no difference between civilian or military aircraft. An airfield just "is". Based on the aircraft's user list (exported = true), alignment (friendly or enemy), and the terrain's limited nation's list (which takes precedence over the games main nations ini)
Aircraft are placed in their parking slots on airfields simply by their wingspan, as stated in each airfiled's ini.
Having more buildings (terminals, gates, baggage containers) is a very easy add via targets ini placement. We just need the 3d guys to build them.
what is really needed, are completely NEW airfields, as the 6 we have don't match any real world locations, civil or military