I don't believe you can change the appearance of the sun or the moon in sky/map maker I'm afraid. If it were possible, there would be some sort of modification or cycle for the moon, just like you can have when controlling the weather pattern in your maps. However, you could provide a modification of the moon which will display in all maps for those who download it, but that's the only way.
If you need any help with modifying the moon in FeralHeart, I will gladly help you with that.
Edit; However, if you're wanting a moon installed to your game, just download one that someone may have made and place it into Local Disk (C:) \\ FeralHeart \\ Textures. Be sure to check that the image named moon and that it is a .png format. It will replace the default moon.
Hmm, I don't know.. Though maybe you should ask them personally. Though, I really don't believe it is possible to change the moon's texture for a specific map or sky.