You just need to create the encryption key correctly, and before you create any users. Maybe the steps could be consolidated, but if you're not sure how to create a proper encryption key, the Key_creator is there for you.
There's no reason why Community Auth can't be installed into an existing application. It's made to be easy to do.
Take the time to read over the documentation. It's worth it, and Community Auth is very easy to use if you do so.
