Once you dumped your own cartridge and the save, you must rename the save according to your game ID (refer to this site: http://3ds.essh.co/ ) and rename the rom to .3DZ. However this only works with savedata equal or bigger than 512kb. Why it doesnt work with savedata below that size is beyond me, didnt find an explanation for that yet. Oh, an dkeep it mind this only works for games using the savetype CARD1 - games that save inside the rom like Pokemon X/Y (CARD2) for example dont allow backing up savedata.