How to set animation clip to BlendTree on Editor?

I want to replace animation clips of BlendTree automatically on Editor. It was possible to get clips in BlendTree, but I do not know how to be set.

I tried in Editor script,

 ---ellipsis---
 UnityEditorInternal.BlendTree bt = motion; //<- successful.
 Debug.Log( bt.GetMotion(0) );  //<- successful.
 bt.SetChildThreshold(0, 1.0f);  //<- successful.
 bt.AddAnimationClip( someClip ); //<- Editor freezes when you do this. This is bug?

I could not find a method of set animation clip. How to set animation clip to BlendTree do not exist?