At work we have lots of models made with Designer and their corresponding DB objects.
With recent SQLDDM v18.104.22.1687 I did many trials importing either from 'Oracle Designer Model' or 'Data Dictionary'.
Instead of writing the difficulties I faced, I taught in asking someone for a 'Guide' on How to convert these models
so I could continue using them with Data Modeler (like designing new entities and the rest of the steps).
Maybe someone is aware of a specific article on a blog on that matter ?
you should be able to use the import feature and correctly grab the data models from your designer repo
if something like an index or foreign key aren't coming through, let us know here