I haven't managed to zoom in on a map as a function in actually reading a book. However it is possible to zoom on graphics files, for example
this map of the Honorverse which I have loaded on my Kobo.
Maps in the book should be included as .jpgs in the "resources" folder I think. If you extract the map as a graphics file from the .epub (which is only a renamed .zip file) then you could load it as a graphics file and zoom on that. This is something I've done on the noted Honorverse map a lot - as I decided to christen my KT by rereading the entire series
- and it's been very useful. You just have to jump between the book file and the graphics file when you need to refer to the map. Not as handy as jumping to a bookmark in the book, but not too difficult or time consuming either. I keep the map file in my "Shortlist" so it's easy to find and load.