Add Additional Fields to Existing Managed Form (Update form)

I think there should be an option to add fields to the managed form after the form has been switched to "managed".

Use case:
Select content type edit form and convert to managed form.
Select a few fields, such as body, publishing options, etc.
Converted.
Install / enable other modules / features.
These new fields do not show up in the manage fields tab.
Unregister the form.
Re-register the form.
Select all the fields all over again.
Organize them in the Manage Fields tab.

Current behavior:
All of the organization (order, tabs, etc) is lost when unregistering and re-registering the form. There is no way to add fields once the form has been registered.

Expected / Desired behavior:
When the form is re-registered, the old organization is saved.
OR
There is an additional option on the rel setting to "add additional fields" once the form is registered.