How to make a list with backend workflows

I have two data type- States (Fields: Name-Text, City-List) and City (Fields: Name-Text, StateName-text). Except for City-List, other fields are already populated.
I am trying to generate City-List using a workflow that will automatically add the city’s name to the list of the State is present in city datatype.

For example, I want to add Phoenix and Tucson to AZ as list.

Appreciate your help!