There are several options to apply reduced allowable stresses. In the STEEL and RF‑STEEL Members add-on modules, you can select the Manually option in Column D in Window "1.2 Materials" and then change the limit stresses in the following columns (see Image 01).
As an alternative, you can create a user-defined material in the material library and specify individual material properties (yield strength, strengths) for it (see Image 02).