We have implemented a new field called Connected Dropdown, which has possibility to add an unlimited number of nodes and sub-nodes, in a hierarchical tree. 

An example would be Location:

Main nodes: Rogaland, Hordaland


    Rogaland: Stavanger, Sandnes, Haugesund

    Hordaland: Bergen, Voss


        Stavanger: Stokka, Storhaug, Tasta

To expand sub-nodes, hover the mouse over a node. To select, simply click.