ofcourse it would be possible that way Pls. note that in Italy & Spain you have to pay on top of the Railpass for reservation. Sometimes cheap advance fares are the better option (carefull not all are refundable)

You can even use the same travelday from the pass both in IT-to go to aeropuerto and again in ESpana, for the cercanias trenes from some aeroporto to the city.

As seewulg already warns: ES is notably very often mentioned here as being a real hassle to get the additional and always needed extra REservations etc. It is not impossible, but heed the warning!

in Italy please note you can not use to Milan Malpensa the Malpensa Express with Interrail/Eurail and to Rome Fiumicino only with an 1st class ticket the Leonardo Express. But you can use normal regional trains to this airports with Interrail/Eurail.