Is it possible to add a watch only address using only the public key? If so how?
I haven't seen one. Maybe you could do it with the python API, I'm not sure if you need the private key or not.
Try adding it to a new wallet then import that wallet as watch only
Watch only wallets only work with deterministic key chains.
The Armory guys have said in the past they don't like the idea of importing watch only addresses, because it is an attack vector. An attacker could insert their own public key into your wallet and you'd have no way of distinguishing it from your own. Then, it would look as if they had paid you, but in fact they had paid themselves.