I'm sure the FID could be change but I've no experience with that though. Anyway, this is not the only, nor the best, option but this is what I've done with an older nuvi 200 which has very limited internal memory making it very hard to accommodate the whole CNNANT2011.30 mapset. Thus, I've placed the local/regional mapset into the internal memory as gmapprom.img--7 map tiles amounting to about 81.2MB. I then placed the remaining CNNANT2011.30 mapset (140 map tiles amounting to about 1.2GB) onto a 2GB SD card as gmapsupp.img. The card could be removed when traveling locally for faster processing but I leave it in all the time nowadays. This approach serves the exact intention you have in mind. FYI.