When calling the data manager's _editRecordForm view, it is possible to specify formName (and also mergeWithFormName), to give precise control over which form is displayed (otherwise falling back to the default of preside-objects.#args.object#.admin.edit).
However, the _addRecordForm view does not allow this, and simply uses the form found by convention at preside-objects.#args.objectName#.admin.add. This means that you cannot customise the add form.
I'm assuming this is an oversight?
Also, _addRecordForm requires args.objectName to define the object, whereas _editRecordForm uses args.object, which is inconsistent. Although I realise that in making it consistent, the old variable name would have to be kept also for backwards-compatibility...