I checked out AVSIM this arvo and took a look at the TweakFS FREEWARE files. The FSX Config editor is something that people may find useful. I had a go and found it to be easy and allot easier than digging around in the config file through a text editor.
Now to be sure it doesn't appear you can add entries, but you can certainly modify those you currently have. Worth a look for what you pay, particularly if your not confident with configs.