You can't add data fields to forms where they don't belong

Only those fields that are available for each form can be used on it. You cannot create new data fields for any form. You also cannot copy data fields from one form to another unless the fields that you want to copy are available on both forms.

Similarly, you cannot add a data field to pages or copies of a form unless that field is available on that page. For example, you can't add a "Subtotal + Tax" field to the first page of a multi-page invoice, because this field is only available on the last page. You can, however, add an "Amount + Tax" column to any page, since this column is available on every page of the form.