Introduce sounds or alter the sounds of bank GM1,GM2 in vs1053 MP3 Player Shield

Hello Arduino and SparkFun developers, I’m working in a project and please need some help. I right now reproduce MIDI files in real time mode, but what I really want to do is alter the sounds bank GM1 or GM2 of SparkFun Shield, to put my own sounds, for example when I call the sound 33 Acustic Bass instead of reproduce the predefined sound, reproduce my Acustic Bass version , It’s possible? Or maybe I don’t need to modify the sounds bank GM1 and there is other memory space in the shield destined to put the user’s files? I’ve been working with Arduino Shield and allows me to play any sound I want, but only allows me to play a sound at a time, is not polyphonic, on the other hand the SparkFun Shield is polyphonic, but I haven't my own sounds. Thanks in advance, L.?