Is there availability for the SCL - MAD - OPO if you look at this as a single itinerary? I appreciate you already have a SCL - MAD booking and, independently, there's availability for MAD - OPO as a separate itinerary, but you may be running into married segment shenanigans. If there's no availability for the complete SCL - MAD - OPO journey then you probably won't be able to add it. You could make a separate booking of course but you then run the risk of what happens if you misconnect in MAD.

To give an example, I have DFW - MAD - BER booked with IB with the long-haul being in Business and the shorter leg in Economy. There were business seats available using Avios on the MAD - BER sector but because of married segment conditions / availability I couldn't book DFW - MAD - BER with both flights in business.

Does that help?