If you had proper custom firmware, this wouldn't be a problem as you could install a decrypted DLC *.cia file with the FBI app. I suppose you could hack your 3DS wth Luma, B9S, and GM9 and install the DLC on the EmuNAND, but at that point you're better off migrating your library and games to the SysNAND and HOME Menu. As was said not too long ago to another Gateway user from this
thread, put that flashcart and its accompanying EmuNAND to rest.