Is there a way of doing this, either direct or indirect? Specifically the files would be back-of-book indexes with indented subheadings and subsubheadings.

There is an old and long discontinued Microsoft filter - MS-DOS Text with Layout converter for Word - which you can find on my web site at Graham Mayor - Downloads and which still works albeit with provisos discussed in the accompanying readme file.

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Graham Mayor - MS MVP (Word) (2002-2019)
Visit my web site for more programming tips and ready made processes www.gmayor.com