The header files declaring Molecules, Residues, etc., are available for download. This allows C/C++ extensions to work with molecular data classes, but not add methods and/or data to those classes. The support for inserting methods and data into molecular classes will be made available some time later in the Chimera "developer" release.
Basically, you write an extension conforming to Python's normal
Once you have a compiled shared library, put it in Chimera's