As far as I'm aware, it can only be done on SD card, though those can go up to 2GB+. Also, I've heard you access it through the photo channel.
Although apparently, Excite Truck can use you own mp3's to play whilst racing...
You can usually get some cheap SD cards from www.amazon.com.
http://www.amazon.com/s/ref=nb_ss_gw...words=SD+cards - American
http://www.amazon.co.uk/s/ref=nb_ss_...o.x=10&Go.y=11 - UK