what's stopping you, get this line in the controller to create this entity?
For example, received $city = $form->get('city')->getData(); to check whether such a city, and create it if not present? DataTransformer need to convert one format to another, as a recording entity not to use.
Joel.Wisoky36 answered on September 26th 19 at 06:36
With shapes in Symphony, you can do anything. The easiest option in my opinion - DataTransformer to write your own.