In beamng? I have no clue, Idk if you can set a custom skybox or something in the world editor, but It might not be possible atm
In the 'ScatterSky' settings, there's an option for the 'moon' texture. Simply use that and point it to a custom texture of yours.
Is there a way to make moon move with time of day setting as I have noticed it is not moving by default? Or would it need rotation of terrain and everything in game world