have lifetime maps in Mapsource, how to put into another Garmin device

I tried to search for my question, but I could not find a answer to my scenario, or maybe I overlooked it as the information on this site is a bit overwhelming.

Anyway I have the latest City Navigator North America NT loaded and available in BaseCamp via my lifetime maps on my Zumo 660. What I want to do is select a subset of the maps (the lower 48 states) and put them on several old Garmin devices (Nuvi 665, Kenwood DNX) that have a 2gb map size limit. I guess basically how to create a unlocked GMAPPROM.IMG file. Last time I found and downloaded a unlocked GMAPPROM.IMG file which worked great.

Thanks.