Yes this can be achieved with a little work, depending on the look you're going for - but essentially, all it requires would be a large, flat and open area of ground segment, with whatever type of fence or wall you'd like as a border for your map. It's all in your creativity though really, the placing of hangars, vehicles, planes. A lot of which can be found around the forums actually! You can check out
http://fpsfree.com/ and
http://fps-files.com/ for some cool media for an Air Force Base.
Might want to specifically check out
Cosmic Prophets page for some really neat things, theres an actual hangar you can construct, along with other entities!
http://fpsfree.com/featured/CosmicProphet/CosmicProphet.htm
For a little inspiration and somewhat of a showcase that this sort of thing can in fact be achieved, you might want to also check out a game being produced by
alimpo83 called Days of Infamy, on the second page in you can find a few screenshots of an Air Base he was working on as a level, very well done and might help show you just what is possible; or at least give you some ideas.
http://forum.thegamecreators.com/?m=forum_view&t=156580&b=25
Other than that, it's more of just opening up
FPSC and trying out a few things. Get an idea in your head, and go with it; just test out different things and see what can be accomplised. I'd say that for the most part, an Air Force Base
could be made. You might want to have in mind factors such as maintaining a strong FPS so you don't have lag, but free, downloadable mods for
FPSC are available such as Project Green and Fenix Mod just to name a few, that can vastly improve the leeway you have in level design, in regards to keeping the lag down.
Also things to keep in mind are making sure the lighting looks realistic to an outdoor setting; if you have a skybox with a sun, put a farely significant light in your level, high above the ground. Perhaps 3 or 4 layers above your ground segment, with a yellow-ish tinge.
Other than this, it's really all up to you! The best way of understanding what can and can't be done is always, just simply, trying it out. I hope it all goes well for you, I strongly believe with some time and effort, this can surely be done
Cheers
Life would be much easier if I had the source code.