Add ability to copy other subfields like the reuse existing field for Drupal core

Try the clone feature and see if that satisfies what you're thinking? You would see it when you add another custom_field. It will show other custom_field instances from any other entity type and it will copy all the storage and widget settings over.

Thanks! That isn't quite what I meant, that copies all values from another instance, I want to copy just individual subfields.