You can book a ticket out of Interrail (Train,Bus,Flight) or you take the Ferry from near Rome to Barcelona

Have you thought of going from Paris to Barcelona (TGV 6hrs40): great city and worth a look; then along the Catalan coast to Marseille (6hrs), continuing beside the Med to Nice, cross at Ventimiglia into Italy for Gerona, Turin,or Milan. Very scenic trip- I did it years ago! Depends on how many days you plan to travel.