For that kind of work you need Photoshop or Photoshop Elements or some other external editing program.
The way I understand your question is, you want to add faces to the People («face detection»), is that correct?
If it is that what you want you can do it like that:
In the menubar (Photos - People - Draw faces region) then you can draw a rectangle around the face and give it a name.
You can then have a look at all your detected faces (View - People)
Daniel (from Switzerland)