Funny, I was just wondering exactly this same thing the other day and thought about posting this same question here. Unfortunately I don't know the answer.
If this isn't possible, then as an alternative I do know that (even without maps+) you can create a KMZ file with I believe up to 500 quantity of 1000x1000 pixel (anything larger than that will get compressed and render at low-res), non-progressive-scan jpeg tiles. See this page and this page for more info.
The downside of KMZ custom maps is the max zoom level you can zoom out to is 120ft map scale, anything further out than that and the imagery disappears. Whereas JNX tiles appear at up to 2 mile map scale.
Another downside is that it's tedious to make the KMZ maps, as you have to do it all manually, positioning and scaling each tile one at a time in Google Earth. I'm not aware of any automated tools for it. Probably fine for small maps with only a couple dozen tiles at most, but a massive time suck if you want to make one with hundreds of tiles.
The user above says there is a limitation of 5 KMZ maps on the device, I'm not aware of such a limit but I've also never had more than 1 KMZ file on my watch, so I'm not sure.
Researching this deeper, I found this Garmin support article which, at the very end of the article, seems to imply that Basecamp can be used for managing Maps+ satellite imagery files (they use the example of using Basecamp for copying/pasting imagery files from one device to another device, as long as both devices have Maps+ subscriptions).
So it does seem that Basecamp treats Maps+ subscriptions the same as they treated legacy Birdseye Imagery subscriptions, thereby giving you ability to transfer and activate JNX files on a device with a Maps+ subscription.
I'm also interested in reusing jnx files. Another way to use large maps is to use rasters in IMG format but it overlaps and covers the vector IMGs so I was unable to show them together
Con la herramienta MOBAC o con SASPLANET los mapas KMZ se pueden hacer fácilmente de muchas fuentes. El problema está en la limitación que impone Garmin para utilizar un número de mapas en el dispositivo. Supongo que a ellos les interesa cobrar las suscripción. Una pena.
