Can you merge fields into header or footer of template?

Yes, you can also merge fields (including images) into the “Header” and “Footer” of document just as you can merge into main body of document. From menu bar at top of Google Doc, click on “Insert” and then click on the header and/or footer to insert into document into document if not already visible. If you do merge a field in this area, the field value or image will show on every page of document.