That's doable through reference selectors. You'll have to build a second entity , linked to your first (many to one association) with a string or enum attribute which would contain your dynamic selection.
Then you would create objects of the second entity for each dynamic entry, and link that entity's string in a reference selector.
Hope this helps.
If I understand correctly, you would like to have two dropdowns: when you select a value in the first one the values available for selection in the second drop down change dynamically. Is that correct? That is possible in Mendix. In the properties menu for a 'Reference Selector' widget there is a tab called 'Selectable Objects.' In this tab, you can add constraints that will change what objects are available for selection. You could write a microflow that will retrieve only the objects you want based on the value of the first reference selector. You could also use XPath to achieve a similar result. On your first reference selector, have an on-change event that will refresh the entity so the microflow/XPaths will re-run.