A head consists of one entire model. The hair isn't a separate model as in other games. So, if you wanted to add more hair options for the player, you would need to add new heads.
The basic steps for adding a new player head are to make the new models and textures, add new rows in appearance.2da, heads.2da, and portraits.2da, then ideally set up an installer with TSLPatcher to handle references to these newly added rows correctly.