The list of Mat_grade values is stored in the SpecGen-Settings.mdb file. This file is stored in the SpecGEN folder in your project. For example:
C:\ProgramData\Bentley\Plant V8i\Bentley Plant V8i Projects\SAMPLE_IMPERIAL\SpecGen
You should be able to edit and add the ASTM A106 and a the description here and then see it in your project specs.