Located in the star subdirectory of your STAR-CCM+ installation directory, the props.dbs file is essentially a text file, and can be modified with any text editor. However, since this file is the source of the material database, it would be best to make a copy of this file for modification, and keep the original in a safe place.
Open your copy of the props.dbs file in a text editor.
This lets you add new materials as well as modify material properties.