I'm trying to extend this bundle for more then week now. This is really PITA. I think this Bundle is not supposed to be extended. If you want to change some more than simple configuration or template, you are pretty much screwed :/
If you don't want to code forum from scratch and use this bundle probably the best solution is to download code and put whole bundle in src folder, and then modify it, instead of trying to extend this bundle. This is not nice or elegant solution, but probably the simplest one.
@reece PHP allows, but we have here something more than only PHP e.g. Symfony2, Doctrine2, this specific bundle.