I don't understand really what your plan is.
With Iberia Plus, online you can only book single or return flights. If you want an open jaw itinerary, just book two one way.
-NYC to Mad,
And Barcelona (by the way it's BCN) to NYC
If you want a stop over on your return from BCN to NYC, then book 2 singles: one from BCN to MAD (I advice you to get a Air-Shuttle Avios redemption) and then MAD to NYC.