This option is not available in the dropdown widget. 

You may check this widget available in the appstore. https://marketplace.mendix.com/link/component/111237

If above does not work, then only workaround is to use an entity instead of enumeration. The entity will have two attributes like an enum. Fill the entity with enumeration data. Now link it to your main entity. This way you will be able to use reference selector to select objects of that entity (used as enum), and in reference selector you can constrain data using xpath to limit selectable objects.