This paper give hints on how you can store additional information / own data section in a Library Part.
As written in the Library Part Database, a Library Part is built from independent sections. Every section has a unique four-byte identifier. There are predefined IDs which correspond to the sections owned by Archicad. Other IDs are free for use to save your own data.
Third party products which create Library Parts often use this possibility. Good examples are the SketchUp In-Out and Google Earth In-Out add-ons, which creates Library Parts representing the models from SketchUp and Google Earth.