If you check the commands possible, I'm afraid... you can't read or change the contents of an SD-card using this module + Arduino.
Most commands are used to pick the right song and start it, others give you the possibility to change volume etc. You can apparently also query some... features.
But none of the commands actually give you the possibility to read the name of a folder, read x bytes of a file or create/append one.
I guess the player is just a player, if you want to change a file on SD you probably need to do it with a computer.