It's difficult to do anything with Penumbra: Overture, as it requires using old software and learning a great deal before anything useful can be made. This documentation is only left here to give insight for those fiddling with the source code for Overture and the HPL1 engine.
This wiki contains all the information you need to get started with modding Penumbra: Overture and the HPL engine. Before you begin remember that to work with the engine you will need experience with using a 3D editor, if you have none or little it is recommended you start learning one before trying to use the HPL engine.
All game specific files that gives objects specific behavior is based on XML files, it's good to have some very basic knowledge of how XML files are structured and function.
The HPL engine uses a C/C++ like script language called Angel Script, as such it is a good idea to have some basic programing experience, no more than having done an introduction course or similar is required.