One way would be to check the "Different first page" option in the Page
Setup dialog box, and then, in the main header, insert one or two
border-less text boxes positioned so that they force smaller margin(s) on
subsequent pages.

See also http://sbarnhill.mvps.org/WordFAQs/Letterhead.htm which does the
opposite: the article describes how to use a text box in the first page
header to create a wider margin for the first page.