I think about it since a long time but i also think that will need to make tests before saying that it can be done as GP4 use a special weather generation system.
if i well saw it, the files to change are some jams files for cloud colors and tga files (or tex, can't remember) with transparency map and they are all in the same folder.
I will have a try myself but not before a week as I'm on hollydays
Bye.