Custom Module - adding new fields - CreateBackEnd

I don't quite understand how to add a custom field in the CustomModules' Definition.

I have a Custom Module called COMPANIES. I built another module (module builder), called STATES.

I want the companies backend to display the STATES dropdown using the companiesdefinition.cs (createbackend method).

I want the dropdown to select the ID of the states and save it to a field that is a varchar string (mapping)..then when a user edits the item, they would see the selected state.

is this possible?

Using the same module mentioned...can i also get an example of an image field that stores as a string (location of the file)?