The following paragraph from the article about the Insert Text field that Stefan gave you applies to the most recent versions of Word as well. Otherwise, you simply insert the text.
"WinWord 2002 and 2003: In the Insert>File dialog, click on the little arrow attached to the Insert button, and choose "Insert as Link." This will insert the entire text of the file as an IncludeText field."
Note that this will not pick up headers and footers and will use the styles in the destination document if the styles are present in both documents.
For headers and footers, look into the **StyleRef Field**.
For yet one more warning on using Master Documents, see: Master Documents "Feature".