Does an add-on exist that will make a contact's Activity page show related Microsoft Word documents?

No add-on is required. You simply need to create appropriate items in the Journal folder. Each item should contain a shortcut to a Word document, and the Contacts box at the bottom of the item should contain a link to the specific contact. With a contact open or selected in a folder, you can choose Actions, Link, File. After you select the Word file that you want to link to the contact, Outlook creates the appropriate Journal entry.