Apparently you are asking for county to be added, although many states don't require that. Entering the ZIP Code often uses a USPS database to determine what they want to use for city, not the actual city itself. (Many of my clients have Scottsdale or Glendale mailing addresses, but they actually live in and pay property taxes to Phoenix.) If you have clients who don't know what county they live in, try voter registration websites.
Bob, We had this feature in Drake Tax and it worked well. The program would populate the city, state, and county on the Drake input form. The Drake program handled the problem with multiple cities using the same zip code. There was a zip code in my area that was used by 3 or 4 cities.
