Usually we duplicate the entire body mesh to create a blend shape for the head, but what if the object is heavy? Having few duplicated object will kill your scene.
The solution is to separate the head (extract), then duplicate and do some blendshapes, keep the first duplicated as the 'base' rather than the original mesh because we can't use original mesh as the target due to vertex order change. As soon as blend shape is ready, create a blend shape to the 'base' so it stores information. Then apply wrap deformer to the original mesh.
I know it's killing the scene by computing the calculation. But the trick here is GO TO INPUT AND CHANGE THE ORDER OF THE WRAP BELOW SKIN CLUSTER. Therefore the wrap deformer is no longer calculated after the skinned deformation. This will dramatically improve performance and resulting cleaner construction history. However another method is by using combine and merge verts, which leave ugly construction history. I prefer to use wrap instead.