Could you say a bit about where you are at in terms of editing a Vassal module? As Mark suggested above, a good starting point is to watch Video tutorials on module development. Also, read the Designer’s guide (PDF), as well as relevant posts on Module Creation.
If you’ve watched those, or acquired some understanding of editing Vassal modules in one way or another, then - as Mark suggested - you could add a Text Label trait to your marker pieces or some prototype that those pieces use.
This will add a context-menu (right mouse - or Mouse-2 - click on a piece) entry that allows you to add text to a marker.
Have you tried to contact the module author of Highway to the Reich: Operation Market-Garden 17-26 September 1944 – 2nd Edition @imikem? (click this tag and press the Message button). Perhaps “MM” will add what you need. It seems the module Highway to the Reich: Operation Market-Garden 17-26 September 1944 – 2nd Edition is actively maintained (last update was 2 months ago).
Yours,
Christian