Yes, UV mapping. You can use free ones like Blender or Ultimate Unwrap (paid) since it can open and save skp files.
But to texture that way you need to use something like Substance Painter or similar free ones like ArmorPaint, Blender can do it also but baking is not as good as Substance Painter or ArmorPaint.
For any of them you will need to export your file to *.obj or *.fbx to texture them and after that, you will need to convert that model to skp to be able to use it in your scene.
