I'm not sure what you mean. "Apparently, these headings were not in heading styles but rather in other styles that had been assigned outline levels in the original document but don't have them in the new document."
I did not say ""Apparently, these headings were not in heading styles but rather in other styles that had been assigned outline levels in the original document."

I set out below what I did to create the Headings but the question I asked is. "Is it possible to somehow or other select all of the headings that still exist in the old document and get them onto the Navigation column?

When I create a new paragraph or chapter heading call it what you will.
I type the heading, hit return so that it stands alone.
Then I open “Change styles.
I then select the heading by stroking my curser across it.
I open Change styles and click onto heading 1 which transfers the heading into the column on the left of the document.