You can use a separate file as the header source.  The easiest way to get to
it if you are using one of the later versions of word is to add the original
Mail Merge Helper to a toolbar (or the QAT in Word 2007).  In the earlier
versions, you do this by selecting Customize from the Tools menu and then
select the All Commands category and scroll down through the list of
commands until you come to the Mail Merge Helper item and then click and
drag it onto a toolbar.  When you use the Helper to create a mail merge, in
step 2, select the "Header Options" item from the "Get Data" pulldown.