Of course this is possible. There are ferries from Bari to Korfu. They are operated by Anek Superfast and Grimaldi Lines (if that’s your question). 

It’s just not free with a normal rail pass (if that’s your question). But you may get a discount. Here’s an overview: Ferries in Europe with Interrail | Interrail.eu

But there’s also a greek island pass (if that’s your question): Interrail Greek Islands Pass | Greek Islands One Country Pass | Interrail.eu