The only tool you'll need is GMax/Max. Apply the texture then, in the modifier list, go down to UVW Mapping and select the radio button for "speherical" mapping. Thing is, you're likely going to need a texture which is made to be applied to a sphere. These are usually wider than they are tall and either get progressively more distorted the higher or lower you go on the image or have less detail in that range. Like this. Things get crunched at the top and bottom of the sphere in sphere mapping. It really depends on the texture, though.
Just to be completely goofy about it, there is another way you can map a sphere in NWN, and that's by applying a transparent texture to it (UVW mapping doesn't matter in that case) and then apply a sphere map as an environment map. When I say "sphere map" in this context, this is the type of texture I'm talking about. Nomatter where you view the sphere from, the image will look the same. You'll want the surface to all be the same smoothing group, though. If you distort the sphere at all (again, make sure all faces are in same smoothing group), you can achieve really neat material effects like this.