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
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?
(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.