You only use RAD ALT as the DH /AH, if radar altitude is the published minimum for the approach. To clarify your question, I think, you would put an MDA baro altitude in the baro window. My company actually uses a DDA or derived decision altitude, we add fifty feet to the MDA altitude for non-precision approaches.
You would also use the baro field on the MCDU for a category 1 ILS. Category 1 ILS minimums are predicated on baro altitudes.
RAD ALT is used only for CAT II, and CATIIIA and CATIIIB approaches, where radar altitudes as Decision Heights or Alert Heights are published.