You may need to modify the code in SkeletonRenderer.cs for that depending on what you mean.
But that alone may also not achieve the effect you're looking. One gotcha is that the default skeleton shader (like most sprite shaders including Unity's standard ones) doesn't use the depth buffer like normal 3D shaders would.
If you're so inclined to read, this may be informative: http://esotericsoftware.com/spine-unity#Rendering