Can I modify the built-in scripts?
Yes, you can open any of the scripts in Script Editor and modify them. If you do, you should make a copy of the script first so that you can go back to it.
Can I create new Folder Action scripts?
Yes, but you cannot create a new folder action. In other words, you can create new scripts that respond to open, close, add, remove, and move window events. But you cannot, for example, create a folder action that is invoked when the folder is renamed.