You can use the Insert File dialog box to Insert as Link; in the dialog box,
select the file to insert, click the arrow next to the Insert button, and
finally click Insert as Link. This creates an INCLUDETEXT field which
displays the external document (and you can update the field to reflect
changes made in the original).
For more on INCLUDETEXT fields, see
http://word.mvps.org/faqs/tblsfldsfms/includetextfields.htm.
If you are using Word 2007, here's how to display the Insert File dialog
box: On the Insert tab, click the arrow next to the Object button, and then
click Text from File.
