You can use psas code or batch command in the idev file, if any, is being used for the dynamic simulation. Consult PSAS guide and/or API guide for the PSSe.
Unless we're talking about very old version of PSS/E I'd highly recommend using Python instead. ierr = load_chng_4(i, id, intgar, realar) is what you're most probably looking for. See more details on page 2-36 or 724 in your API.pdf