You can, but I haven't tested it yet myself. I recall you need a Fat32 partition and you will want to select USB-NAND from the triiforce menu or use the auto usb loading .dol.
dutycall: I actually tested Tetris party on mine the other day and it worked, but swords & soldiers won't load, it actually freezes. Triiforce is not foolproof. There is actually a game compatibility list on the net, but it's fairly short. Just google "triiforce game compatibility" and it's the first thing that comes up.