Create CCK Module or use Rules Module to update fields

I have a CCK field called "City". It is set up as a dropdown and has a list of allowed values. I created a CCK text field called "New City". If the City dropdown doesn't contain the City where the event is, the user can type in a value into new city. At that point, I want a trigger that will 1) Add the new value to the allowed values list, 2) Update the value of City to be the New City Value and 3) set the New City Value to NULL. I installed the Rules module to try and set this up but it's not working. I'm not tied to a particular methodology, though this must be a common issue that could use a module to handle it. Bob