The ITP subdocument is able to handle master and subdocuments that have been made using the Microsoft Word master/subdocument facility. The subdocuments are included as if they were included using an __INC(…).
Add the setting
ITPWFWSUBDOCS=Y
to the configuration file to enable this support.
Due to the way in which ITP handles styles and page settings the resulting document might differ from the document which Microsoft Word would show when examining or printing the merged document.