Yes, it is possible but you will need to hex your client and replace the name of a gravity map currently using the blue sky effect with your custom map's name. This will effectively remove the blue sky effect from the gravity map and place it on yours. Obviously you would want to choose a sky-based gravity map that is restricted from players. One of my video tutorial videos covers how to do this.
When i mention wither in my video thats actually a custom map already covering a gravity based sky map. I failed to explain that this custom map is my own map that is replacing one of the gravity sky maps and I covered wither with another of my custom maps to demonstrate the sky effect.
Refer to my signature to quickly access the BrowEdit Video Tutorial page.