I have successfully added my campaign info to PCGen, but it was a tedious process. However, I know there is help available from the PCGen forums, but didn't bother to ask for it (I'm stupid like that). I was able to add a custom class, custom magic items (and a whole new magic item type) and new feats. These new items appear in the UI as a separate source that you can load, just like those from the various publishers.
PCGen is a great program, IMO, but I do find the hassle of adding new material to be a bar to entry. If you want any specific help, let me know, or stop in at their forums.