Simply replace the box mesh of any default sky and rename with a sphere, if your texture is panoramic or spherical mapping you can adjust the uv's to suit in any modeling app. Use the default sky shader. scale of the mesh shouldn't make any difference. Make sure to save the .x as text file so you can name the texture reference in this.
The Galaxy seed sky has a few transparent layers and horizon masking, it uses a scrolling texture shader which wont suit a static single texture sky.
