A Sky Box and a Sky Shpere are 2 different creatures really. In a Sky Box you have multiple images placed on the inside of a cube which has certain advantages and disadvantages. In a Sky Sphere you will usually have a single image placed on the inside of a sphere.
I can't think of a way to get good results taking the images from a box and placing them on a sphere. Good Sky Sphere images require good images.
I use the DB commands to make an inverted sphere rather than create it in a modelling program :
` create a skysphere and project a starfield on it
MAKE OBJECT SPHERE 50, -50000
TEXTURE OBJECT 50, 500
Iv'e attached a starfield image I use as the base for my sky system as well as the base 'blue sky' image I use. Experiment with them and you'll see they can be easy to make and look good

I use sets of nested spheres of decreasing sizes to create my skies. Rotating the spheres can create very nice effects.
Edit: make sure your camera range is set so you can see the sphere

You may have to flip your Images to get them oriented the way you want.
Xgame101
1024 Studios