You should just be able to press Ctrl + Alt + N on any bone to add a new child bone. If that's causing a crash try the latest version of Brawlbox (v0.77).
Yeah I already tried both of those options but none worked. Basically Brawlbox v068b is the only version that can edit the pac file of a character for some stupid reason! Anyway thanks bro for trying to help me and no worries because I'm not giving up that easily.