Unless the document specifically allows for a power of appointment like you describe then you are out of luck. However, practically speaking nothing stops you from getting the advice of the nurse you trust as part of your decision making process....she would just not have the authority to make decisions but could clearly provide valuable input to get you to be in the same place you want, no?
My answer is not intended to be giving legal advice and this topic can be a complex area where the advice of a licensed attorney in your State should be obtained.
If your father is not capable of signing a new designation, then it can't be done. It doesn't matter what you, the other alternatives, and his wife want to do -- if he is unable to state what he wants, then it can't be done.
As a general rule, it is not a good idea to designate the provider of care as an agent under the medical power of attorney, as this creates a potential conflict. The agent is required to act as your father would act in making medical decisions, whether those decisions are in accord with good medical practice or not. The nurse would have conflicting loyalties, since her duty as a nurse is to comply with good medical practice.