With the file open in the text editor, scroll to a set of data for any material, preferably one that belongs to the same phase as the material you wish to add.
Select that entire set of data and use the Copy command.
Then paste that data into an appropriate location, and change the entries to reflect the data of your material.
To complete the addition of a new material, you need to enter data for your material in the proper positions. The details of how various types of data appear in the file are described in the section on the material database format.