First, you don't want to turn this into an .img file, that involves converting raster data to vector data and will not give you good results.
I think your Monterra supports 500 raster map tiles (in .kmz files) but am not positive since I haven't used one myself. I know my Montana supports 500 tiles though and have read the Monterra is the same.
But the custom map (.kmz) format just won't allow enough coverage for a map as large as you want. I only offer local maps on my own website, however the Mobile Atlas Creator software I use should be able to do what you want, if you can find your desired map on a server somewhere and create a custom mapsource to access it. Mobile Atlas Creator (MOBAC) is a free open source program that runs on all platforms and can make maps for just about any device and app, including your Monterra.
Custom mapsources are more complicated than I can explain right now, but the MOBAC docs have examples of custom mapsources and you will find more examples with some Google searches.
See my MOBAC page here, it includes a detailed tutorial:
https://boydsmaps.com/mobile-atlas-creator/Section 6 goes into detail about making Garmin maps. See the advanced section about getting around Garmin's tile limits by creating .jnx files. The catch is that you need to have an active Birdseye subscription for that to work however.
You could also use the okmap software described there to create custom maps and .jnx files from other sources, if you don't want to use MOBAC. But Okmap is not free.