I've been using this program for a couple of years now and make great use of it.
I'm by no means an expert with this program, but I don't think there's any way to add/delete tracks once on your gps.
One possibility may be that you have to create a few images on separate SD cards and carry them with you. Does your 276C have the ability to have more than one gmapsupp or gmapprom? With the units that have that ability, you name the images: GMAPPROM.IMG for the primary mapset GMAPSUPP.IMG for the second, and GMAPSUP2.IMG for the third mapset (Internal mapsets only) If on an SD card, they would be GMAPSUPP1.IMG, GMAPSUPP2.IMG etc.