Adding Airfields to Mission Builder
You can import numerous airfields into CFS2 that you can fly from using Free Flight but you won't be able to use them in Mission Builder without a little extra effort.

To add new airfields for use in  Mission Builder you must first add them to the airbases.dat file in the folder INFO. The airbases.dat file is a simple text file and can be opened with Notepad.  While much of the information shown there for the existing airbases can be copied for your new entry, you do need the correct longitude, latitude, altitude and heading. I have found that it is best to take the longitude and latitude from the center of the airstrip.

To get that information go into Free Flight and select the new airfield you want to add to Mission Builder. As you’re sitting in the cockpit, before you start your engine, press ALT.  Then select Views, View Mode, Top-Down. This will place you well above the airfield looking down.  Your plane will change into a red cross.  Press the “Y” key to enter slew mode.  This brings up a line of text across the top of your screen with the exact longitude, latitude, altitude and heading. Slowly push forward on your joystick to move the cross to the center of the runway.  When you get to that point write down the longitude, latitude, altitude and heading as shown on the screen.

Do this for every new airfield you want to add to Mission Builder then you can exit CFS2. Once you have the data you need, open up the airbases.dat file and add the new information using Notepad. Scroll down to the bottom of the file and copy the last entry.  Paste it in below the last line of text but above the little square box symbol.  Now replace the information in the copy you just made with the new information you wrote down from Free Flight.  Be sure to change the name of the airfield.  You must also change the Runway and ID numbers.  Simply add one to whatever the last numbers are in each category.  Make sure that the last thing below your new entry is a couple of blank lines followed by the little square. 

Save the modified file and open Mission Builder.  Your new airfield will now show up for use in designing missions.  See below:

[runway.92]                You will need to add "1" to this number
id=8046                           You will need to add "1" to this number
name=Midway                You will need to change this name
base_lat=N28 12.31          You will need correct longitude and latitude
base_lon=W177 19.81                    "                   "
base_alt=0                  You will need the correct elevation
heading=254                You will need the correct heading
allegiance=0          ******** from this point down you can simply
units_across=2                 copy from the last entry *******
takeoff_start_pos=-450,0
takeoff_stop_pos=1600,200
landing_start_pos=5000,350
landing_touchdown_pos=550,0
landing_stop_pos=-250,0

Some things to be aware of.
•      Runway numbers must be sequential starting with [runway.0].  If you skip a number or otherwise break the sequence all runway entries below that point won’t show up in Mission Builder.
•      Runway id numbers cannot be repeated.  Some addon runways come with an airbases.dat entry for you to use and already have this number plugged in.  Do a search for the number using the “Find” feature in Notepad before adding that runway.  Duplicate id numbers will cause squirrelly things to happen in Mission Builder. 
•      The airbases.dat file must end with a couple of blank lines followed by the little square box.  If you enter a runway below the little box it won’t show up in Mission Builder.
•      If your plane is sitting too far back, off the runway in the grass, you can adjust the “takeoff_start_pos=” line.  In my example above the takeoff_start position is 450 feet short (-450) of the longitude and latitude settings that you entered.  Since we are using the midway point of the runway as the centering mark you should never have a problem.  If you were too far back you could reduce this number to something like –250.  Now you would start 250 short of center.  If you want more runway to take off with, raise the number to something like –1000. Don’t worry about the rest of the takeoff and landing stuff.  It isn’t worth tweaking.
•      The “allegiance” field matches the country numbers in your country.cfg file.  Leaving it at “0” will mean your airbase is neutral and will show as a gray “X” in Mission Builder.  You can then control allegiance by using Front Lines.
•      There is a maximum number of runways  that you can add to the airbases.dat file.  For me it came at 241 airbases or 64kb file size. 
•      You may find a slight variation between the heading you entered and how you are lined up on the runway due to magnetic declination.  You can tweak the heading in the airbases.dat file if needed after you make the initial entry.

There are other ways to get the longitude, heading, etc. information that you need but the method I described here has worked well for me.