from texture2D to cubemap

Hi ,

I created a texture2D object from an image I pick from gallery (I'm working on an android app). I have a Material called "main" in my Resources folder , and it is a Skybox/Cubemap material. If I want to set the material texture in my script, I need a Cubemap texture, not a Texture2D texture. So, my question is: how can I transform my Texture2D to Cubemap? I post a piece of my code to better explain my problem (I don't know what to write in "//from mainImage to cubetex ?????" part).

 private Texture2D mainImage;
 private Cubemap cubetex = new Cubemap (2048, TextureFormat.RGB24, false);
 private Material mat;
 
 mainImage = new Texture2D (www.texture.width, www.texture.height);
 mainImage.SetPixels32 (www.texture.GetPixels32());
 mainImage.Apply ();
 
 // from mainImage to cubetex ?????
 
 mat = Resources.Load("main") as Material;
 mat.SetTexture ("_Tex", cubetex);