There's several options for you.
1. You could choose any cruise line sailing in Europe. Find an itinerary you like. Fly over, do the cruise, tour Italy etc, and then book the QM 2 for your return cruise to NY.
2. Book QM2 for an eastbound crossing - book another cruise in Europe, and do your tour, and book QM2 on the westbound back to NY.
You'd just have to check on the dates to see if the QM2 is doing those 6 day crossings on the dates you need.
There's a possiblity of looking for other lines doing repositioning cruises, but that time of year, it's not likely.