Making the robe part is easy. If you try to make the robe and hood all one thing, though, you run the risk of clipping - and trust me, it'll clip. You'd have to make a seriously gigantic hood on the robe model to accommodate most heads, and some of the large hair female ones would be impossible to cover. Considering that PC's heads dynamically look around based on people speaking and such, it'll get really ugly.
The most common solution is to make the robe and hood separate. Now, it isn't possible to make a hood on the helmet layer continue to display the chosen head; this is why the hood model has that blank white faceless texture under the hood. Also, the 'hats' we have are either cloak or robe layer and don't move with the head.
So, it comes down to this: What are you going to sacrifice to make it work? Are you going to have a hooded robe that has a lot of clipping issues, or one that is two parts and doesn't show the face properly?
Personally, I'd be happy with more hood choices; I've seen a few that are hood and face mask combined with a generic bland face showing through the middle that look nice, I don't know why CEP didn't pick it up.