Could you add the 73783528, 24 .in x 48 .in base extension on the 075100015, 48 .in x 60 .in Fundo Riolito shower pan to make a 48 .in x 84 .in sloped shower pan?

Yes, (075100012) you had wrong # for the, 48 .in x 60 .in Fundo Riolito shower pan. This is a four way sloped pan and at the edge the slope starts from the corners so a sloped extension would fit perfectly.