How to select an address field city from dropdown list?
Consider using the Select (or other) module. Here is an excerpt about it from its project page.


  Provides a new Forms API element which is a select/radios/checkboxes element that has an 'other' option. When 'other' is selected a textfield appears for the user to provide a custom value.


So basically you could "start" from a few cities (City 1, City 2, City 3), and whenever somebody needs another one (not in the list yet), they just enter "City 4". Here is a sample about it (just replace "Option" by "City" to get the idea):