You can do that in G2 itself (not in G2 Lite though, I think).

To design an airplane in G2, you first select an existing airplane that resembles the one you want to design, to use as a basis for your own design. For instance, if you want to design a trainer start with the PT40, if you want to do a DH Tiger Moth start with one of the existing biplanes and so on. Unfortunately you cannot change what the airplane looks like, except for the paintwork and the size that is.

If you look in the "Aircraft selection" menu you have got a heading that says "Customize aircraft" and some buttons below the list of existing aircraft. Press "Make a Copy" to save a copy of an existing aircraft. G2 will ask you what you want to call your airplane. Then select your airplane in the list and press the "Edit" button. A new menu with a number of tabs for various features of the airplane then opens. I usually take measurements from drawings or an actual model and then input all of them into G2. That can take awhile, especially the first couple of times.

I have made maybe half a dozen new airplanes by now and also like to customize the existing ones to try out other engines, propellers, etc. It is a lot of fun!

I haven't tried editing airfields yet, but I think it works about the same way as the aircraft.

Happy New Year from Sweden!

Krister