You can't do this as there isn't any function like that. But one trick you could use: create two forms, one for post creation and another for editing. When submitting the post update the form ID with the edit form's one using action hook.
Another trick is to use action hook field in the form. There are parameters in the callback function and you'll get the post ID in that function. So check the post ID parameter and if it exists (means it's the edit area), show additional contents on the form. Hope I am clear.