To be honest, I'm not quite sure what it is you want, but I'll give it a go:

• Create your design (don't worry about the margins at the moment - JUST the design)
• Select all (Ctrl+A)
• Cut (Ctrl+X)
• View > Headers and Footers
• Paste the design into the header and rearrange as needed
• Exit the Headers and Footers
• Change the margins to suit
• Save As ... > Template
• DONE

Now you have your design on all pages, as a template, so that it will create a new document every time AND the design is unchangeable except to those who know about headers and footers!

Is that what you were looking for?