Yes, this can be done. You will need a .nds ROM of the game, and you will need a flash cart (if you want to play your hack on the DS), or an emulator (if you want to play it on the computer). I can't give out the ROM (just Google search for it), but I would love to help set up an emulator or flash cart if you haven't used those before.
I recommend this ROM editor made by neltazero: