Like zero said, please read the wiki. You can send information to and from your DS using a small FTP server running on your wireless router. Yes, the cheat database is upgradeable as well. I don't know of any homebrew app that can manually edit cheat codes.
^ That's why I could never get far into LGPE; now *that's* "too different", with so many changed and removed mechanics and species and moves that it felt so fundamentally anti-progress, not fun at all.