Attached Link: http://laps.noaa.gov/albers/sos/moon/moon_8k_color_brim16.jpg

Just in case you reconsider your decision of adding the moon in post, I would just like to say that creating a sphere and mapping a hi-res map of the moon on it can yield really great results. That's always my approach (check out my gallery for night shots with moon present).

All you have to do is simply apply a map to a sphere and, optionally, attach the bump channel to the same image. Usually, I place the sun just behind the moon and make the moon material not cast and not receive shadows. The illusion that it's the moon casting light is perfect. Furthermore, the sun's corona also adds the illusion of the halo around the moon.

See link for a 8192 x 4096 map (there's many more on the web).