http://ds.ign.com/articles/631/631077p1.html
EDIT: According to the Japanese site the date and price of the expansion pack haven't been decided but they will release new information about it on 8/22.
Nintendo will be putting the DS's Game Boy Advance slot to good use for an upcoming expansion pack for Band Brothers. Tentatively titled Band Brothers Zouka Kyoku Cartridge ("Band Brothers Additional Song Cartridge"), the expansion will feature 20 additional songs and will ship on a Game Boy Advance cartridge.
The new songs are being selected with the help of Nintendo's fan base.
EDIT: According to the Japanese site the date and price of the expansion pack haven't been decided but they will release new information about it on 8/22.