I'm pretty sure it's mostly open-source. I'm not sure about that though. If someone would add support for RetroAchievements they would probably have to look at the information on the RetroArch website.
Is it possible that support could be added for RetroAchievements when playing GBA games (and other emulated consoles)? You can find more about them on their website here: retroachievements.org
Do you think that you could just make an OBJ to NSBMD converter? I really need a way to convert my models... and all I have as far as modeling programs is Blender.