Is there a way to convert a MIDI file to MML? If anyone has a program that can do this, please reply with the key, if one exists.

There used to be a program called QMIDI (Windows), but I don't know if it exists anymore. Plus: There's no way to get programs to the SB without some sort of other method to get programs from .txt files on your SD card that if I mention how, it'll probably get me into trouble. What I mean is even if there was, it'd be a pain to get the generated MML file onto the 3DS

Getting a file onto SB shouldn't be much of a problem if you use PetitModem (an auxillary file transfer system available on the website "rei.to").

I completely forgot about PetitModem. Then yes, look into QMIDI and then see if porting your favorite MIDIs works that way.