Ciaoi! No, this is not that easy, and its impossible to avoid all extra cost, but it can be done like this: do NOT use planner of EUrail, best is bahn.de-works for nearly all of EUR, but NOT ESpana-they stick to their own renfe.com.
In IT on Tr-It you have to pay for use the IC=3€ or the frecce=10€. You have to reach Ventimigila, via Genova. You then enter FRance, company SNCF and it will not go fast. The regionale, named TER are without extra, they run via Marseille-Tarascon-Montpellier-Narbonne to Cerbere/Port Bou (just before/after the border with ES). You may have to change in some of these towns, and depending on where you start in bella Italia probably have to overnight there.
Then from Cerbere/Port Bou the spanish broad gauge starts-the local treni there are rodalies and bring you to Barcelona, ev 2 hrs. OR you can speed up but that will cost a lot extra (think of >30€) and use the internazionale TGV/AVE to B.
Spain has no more nightrains at all, it now depends on how much you are willing to spend-but extra traveldays or hotelstays will also add to the bill. Make it a nice trip and take the chance to also see some of these towns.
1/day there is a IC from Barca all the way across norte ESpana t the coast Galicia: Vigo, this will take nearly all day, dont know extra cost, but its likely also 10-20€. Again night to stay.
Next day: 2 treni to Porto, named Celta, also dont know how much extra to pay. basically in ES-just like in IT, you have to REServe a seat in all longer distance treni. You cannot avoid that as the local trains do not go all that way. There is however 1/day a Regional Express=without extra, from Barcelona to Madrid, takes >12 hrs though (the superfast AVE less as 3 hrs).
I think you need at least 3 traveldays to cover the whole trip.
Also see Seat61 for the few possible routes to Portugal.
Rail Europe managed to get Spanish timetables and have a decent planner.
The timetable Cerbère/Portbou-Barcelona can be found here. Some SNCF TERs continue to Portbou and are also included. The MD trains require a 4€ reservation, which can be made in Portbou or of the ticket office is closed, the ticket inspector should also be able to sell them.
The direct train Barcelona - Vigo currently seems to run on Wednesdays, Fridays and Sundays. Train type is indicated as Arco, so a reservation should be €6.50. Beest to book it as soon as you're in Portbou.
The IC Vigo - Porto was reported to have free mandatory reservations. The normal fare is between €5.25 and €14.95 though, so that's normally to cheap to use a travel day for.
