I’ve also tried to edit the existing T_Sky_blue texture to include objects such as buildings, but can identify which bit of the texture corresponds to which area in the game, so my buildings are going on the ground or in the sky.
we use hdrshop here at work to convert Cube Map (Vertical Cross) to Latitude/Longitude sphere.
with some googlefu, you can find the old version that is completely free
so you set up your cross and go ahead and paint on it, building clouds and whatnot, then throw it into hdrshop and it will convert to an image that will map to a sphere mesh
