A lot of the custom .bgw files I use in game (over 150) have come from this site:
http://www.smashcustommusic.com/
The nice thing about it is that there are a massive amount of songs from games, and already set up for looping.
1) Navigate to the game the song you want is from, find the song, and download the BRSTM file.
2) Note that on the song's details page, a value is given for "Start Loop Point". Save that number.
3) Using a conversion program, convert the BRSTM file to .Wav.
4) Open a calculator and divide the Start Loop Point number by 16. Convert this new number to hexidecimal, and save that value.
5) In the WAV -> BGW section of XIMC, there's a box with a default value of 1. Replace the 1 with that hexidecimal value you got.
6) If that check box with a number if checked off, it'll save the output file as "music###.bgw", with the ### being whatever you put in that box. Uncheck the box or else you may save over other songs you've made.
7) Click on the top left button in the WAV -> BGW section and select the .wav file you are converting. In a short moment, it will be ready (unless it screwed up).
Note that you can replace songs while the game is going, but do not replace a song that is currently playing in game. If you are using extra songs (activated using Windower addons), you can simply give them a number that isn't being used (I use 800-899 cuz whatever) and store them in any of the folders listed above.