With the
http://doc.sccode.org/Classes/LanguageConfig.html class you can "add" a path to your includePaths, and then call "store" to make it persistent.
Then you can run thisProcess.recompile() in order to compile your current includePath. Notice you probably execute that code only once, and you probably want to keep your eyes on it, since if you move the classes path you will have to move it on the LanguageConf as well.
You should also know that class its just an "interface" for editing that file: ~/Library/Application\ Support/SuperCollider/sclang_conf.yaml
peace.