our Official Kentico trainer that a page can be designed using either the Design tab OR the Form tab. Not a combination of the both tabs.
The design tab is just that, you set where the content is to be displayed on your page template, you don't add content there. The Form and the Pages tab are where you add content. As Suneel stated, it is best to use a new custom page type with the fields you need added and then in your transformation you display those values of the field.
Personally I use what Kentico has designed and recommended, custom Page Types. I very rarely use editable text webparts.