I replied to your pm thinking you wanted to drop a man-gun into an existing/live terrain, but if you are building a custom terrain, you can add a man gun about anywhere you like. Assign it to the base in the TE or even an existing terrain, and you're gtg. Know that it may show wonky in the in-game hanger. If you want it in all the bases, you should modify the field tile in the OE, and build it for use in your terrain like any other base, a bridge, or any other object.