You can import your cities with Excel into a formaloo and then use the lookup field to look up the cities.
Import your Excel into formaloo: You can import your forms and their responses into Formaloo using an Excel spreadsheet or Google Sheets. Import data into Formaloo
Add a lookup field to your form: You can create relational forms by looking up the fields of a form and enabling your audience to select from those. See how to look up the data of another form.
If you have a few numbers of cities to add, you can add a dropdown field and add your cities manually.