Import markdown drop-down menu into side-nav template

[kevinmcl]

Before he left, our template guy modified a side-nav template (don't know which one) with corporate colors and page elements, and all worked mostly fine.

Now, another group - working in markdown with Pelican - has made an entry page for everybody's docs that includes a top-nav, drop-down menu of product version docsets. https://thalesdocs.com/gphsm/Content/lu ... leases.htm

For our product line, we maintain a version-inclusive docset ( https://thalesdocs.com/gphsm/luna/7/doc ... e_Luna.htm ), but Product Manglement wants us to insert the other guys' menu-of-archived-versions at the top of every page in our current docs.

The color bar at the top of each page has the corporate logo on the left and the Search box over on the right.

We'd like to insert that top menu into the middle area of the blue bar much like it is in the first link, above. The template will let us insert some useless account login thingie (our docsite doesn't use accounts or logins), but nothing else. So, how to get that top menu (generated elsewhere) into every topic/page of the second link, via our Flare project, without interfering with anything else in the docset.
What's my next step?

NOTE: The top menu of interest is for objects (other docset landing pages) that are completely external to the Flare project.
The side menu is in-project ToC.

It should be simple, but this is what happens when you have people working on doc content for several products and one guy doing the template work for all of us. My early, meagre skills with CSS and template manipulation (from the days when I was the onliest techwriter) have long evaporated. Please tell me you've done this, and it was easy, and didn't need any post-build, post-production interference.
If all else fails, we can probably work-up something with Python scripts to do the deed, but I'd rather do it within Flare if possible.