Is there a reason you can't just insert suitable headings at the appropriate place in the document? That way, you wouldn't need to unlink the Table of Contents. To differentiate them from the others, you could include the word 'unavailable' and, if you use a different Heading Style, have the Table of Contents not display the page # for that level.

Another option would be to bookmark each of the ranges for which content is available, then insert a separate Table of Contents for each bookmarked range and, between them, insert your static content.

Either way, no unlinking would be required. Once unlinked, there is no way to relink a Table of Contents - it has to be recreated.

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Cheers,
Paul Edstein
[Fmr MS MVP - Word]