ok, there is another way.

1. Select head
2. Apply the expression
3. With the head still selected
   Object->Spawn Morph Target (give it a name)
4. Look at parameters, in 'Morph', there must be the morph you've just created. Decrease it a little.

It must be affecting only the head (not the other body parts)

Well it will affect the other head morphs but that is unavoidable if they are affecting same points.

Greetings: Candid