Xermo ya gotta believe man!
Okay! I've spent all day trying to use python code to magically make nus3bank files, but then I realized an even easier method.
Smash custom music
http://www.smashcustommusic.com/ can convert brstm files (Brawl Music Files) into nus3bank files (Smash4 Music Files).
Just pick a song you want, then select the drop down menu "Download or Add Template For" and select Smash3ds or Wiiu. This will convert the brstm into a nus3bank file.
BUT! I couldn't find the song I wanted there. So I found a brstm converter some incredible genius made
http://www.lakora.us/brawl/brstm/old.html
This program allows you to input a wav, and make brstms.
BUT! I only had an mp3 of the song!
So I downloaded audacity, edited the song so that it looped properly, And saved it as a wav file.
I then put this wav file into the brstm converter.
Then I uploaded the brstm to Smash Custom Music
Then I downloaded it as a nus3bank file.
Once you have the brstm converter, audacity, and an account on smashcustom music. The process is rather fast.
Mind testing out my custom song on your 3ds?
http://www.smashcustommusic.com/70343
(I would recommend using it as Ganon's victory theme. Oh so satisfying)
Also, you can probably find other programs or a website that converts mp3s to wav files besides audacity. But audacity lets ya edit it so you can have the song fade out or loop better.