How to merge two .unity3d files without using assetBundle

I have a .unity3d file whose source I do not have access to that I would like to merge into my scene. Is there a way for me to do that without it having been bundled into an AssetBundle? The only thing I see for non-bundled .unity3d files is LoadWebUnity() (which I haven't been able to get working), but that will replace my .unity3d scene with the new one, not merge them, right?