How can i enable the "Edit mapping" feature on a custom asset import app

I started an import project from forge create following the Use Async Events API to queue jobs to import objects into Assets tutorial. But when i deploy it, and open the Edit mapping toggle, it says:

This mapping has been predefined by and no new object or attribute mappings may be added. Changes to this mapping may result in unexpected behavior.

However, i want to allow mappings in similar to the way CSV or JSON does. Is it possible to make this happen in a custom import app?

My current workaround is to have a text field in the Configure app dialog instead, but this is way less user friendly. It would be nice to just have the normal mapping form be available.