I might have posted about this before, but the way $:/core/ui/EditTemplate/fields has changed in more recent versions of TW (5.2.1+) and I have been unable to recreate this look of the field editor where there is less of a visual break between the existing field data and for adding a new field.
Pulling the new fieldname into the same table may be unnecessarily difficult because it is not the same as a regular field edit, and an alternative add field can be introduced by tagging it $:/tags/EditTemplate
If you are going to this effort can I suggest a new tag such as $:/tags/field-edit-toolbar that allows the introduction of other buttons with each field editor line.
I have planned to do this myself, so may share back some results.
[edited] I am investigating and see how the change you want to make is not catered for in the new mechanisium. The layout and iteration of the field editors are contained in $:/core/ui/EditTemplate/fields which is itself introduced with the $:/tags/EditTemplate tag. I suggest clone $:/core/ui/EditTemplate/fields and modify it how you want and when finished remove the $:/tags/EditTemplate from $:/core/ui/EditTemplate/fields to remove the default field editor.
If I understand you right, this should be a simple CSS matter. This shows how the field-adding part is pushed down 20 px (after I created the tiddler, saved it, and repoened). Note the tag and the type:
