The material database is nothing but XML. You can get the path to database file using this approach: http://help.solidworks.com/2013/English/api/sldworksapi/get_material_database_paths_of_document_example_vb.htm
Then you need to use XML parsers to modify the attrubutes.
Hi there and thanks for the response!
I see the XML material databases but want to define for instance my own "Tables & Curves" data, not "Density vs Temperature" for instance but "Wavelength vs Refractive Index". I am not sure about storing data like this in the XML file.