You could insert an IncludeText field in place of the address to call
bookmarks from a document saved on a local disc eg

{ INCLUDETEXT  "d:\\My Documents\\Test\\Merge\\Address.doc" address
\*charformat}
in the header for the address and
{ INCLUDETEXT  "d:\\My Documents\\Test\\Merge\\Address.doc" address
\*charformat}
in the footer for the fax number

where the address bookmark contains the local address and faxno the fax
number

Then with an autonew macro in the document's template, based on Dave Lett's
code from another post today, you could unlink the two fields to ensure that
new letters produced from the template are not updated subsequently at a
different location to show the wrong address and fax number.

The following will unlink that particular field. Other fields in the
document are not unlinked.

Dim iFld As Integer
For iFld = ActiveDocument.Sections(1).Headers(1).Range.Fields.Count To 1
Step -1
   With ActiveDocument.Sections(1).Headers(1).Range.Fields(iFld)
       If .Type = wdFieldIncludeText Then
           If InStr(1, .Code, "Address.doc") <> 0 Then
               .Unlink
           End If
       End If
   End With
Next iFld
For iFld = ActiveDocument.Sections(1).Footers(1).Range.Fields.Count To 1
Step -1
   With ActiveDocument.Sections(1).Footers(1).Range.Fields(iFld)
       If .Type = wdFieldIncludeText Then
           If InStr(1, .Code, "Address.doc") <> 0 Then
               .Unlink
           End If
       End If
   End With
Next iFld