They would have to be created.
Tilemaps are still a bit hard to edit in the editor, especially if you are using multiple Tilemap objects for example.
We do have addons that contribute to what a third party editor could offer, such as Rot.js, or even the Advanced Random plug.
It might also be beneficial when working in teams as it would be easier to do edits specific to the layout and not have to share the .c3p file.
I should say the the import shouldn't be destructive. Meaning the import wouldn't get rid of objects that already existed, and should overwrite objects with the same name.