I'm loving yasu right now. First the R4 kernel customizer, now the real bdxtool (The one that came out just now 6/30). I just tried it out, it's an all-in-one program for Daigasso! Band Brothers DX; you can add songs to your custom songs list, extract songs that you created, and best of all (For those of us with a large collection), it serves as a converter for the .bbs files that were used in the first game. I'll guide you through yasu's site, since I know he dislikes it when people directly link to his files.
1) Here is his home page:
http://hp.vector.co.jp/authors/VA013928/
2) Click on the E.T.C. link, then the NintendoDS link.
3) Click on the link on the line "? bdxtool(08/06/30 00:30)" to download the latest version
4) Download it and extract the zip file's contents into any directory. The readme is entirely in Japanese, and the program itself is bdxtool.exe
Now for the program itself:
1) Open up the bdxtool.exe file (Luckily the interface is in English for this program)
2) Choose File -> Open SAV
3) Locate your save file and choose it in the window that pops up.
4) Now there should be rows of hyphens (100 of them to be exact), unless you've been creating songs, of course. Click on one of the hyphens to highlight the row.
5) Now the import button should be enabled, click on it, and find the .bbs or .bdx file you want to transfer to that slot.
6) Repeat with any other songs you would like to transfer. If want to extract a song you created, then highlight it and click export to begin that process.
7) When you are done, click File -> Save As SAV to save your new sav file. I suggest making it a slightly different name from the original for now to prevent the new file from overwriting it, you can rename the new file when you have backed up the original.
8) When you start up your game, the songs you just transferred will appear in the main single player game, sorted the same as all of the other songs (Alphabetically)! The songs themselves will be stored in the song creation mode's list. It seems like you can edit, copy, and delete the songs from within the game. The songs I converted from BBS files seemed to keep their quality intact, but I noticed that my star level was reset to one star (I'm not sure of whether or not that was just a fluke).
In case you don't have any bbs or bdx files to test out the program with, here's one to start you out with:
http://www.youtube.com/watch?v=0wm0XBm2qfg (Click "More Info," it's the second RapidShare link)