Yes, this should be relatively easy to do. The Amtrak will arrive on the same level as the MBTA train. In fact, there are only about 12 platforms (maybe 13) at South Station so your walk will be minimal. There is ample room in the main room of the station (which is just through the doors at the end of the platform) where you can find books, seats, restaurants, and yes, ticket windows.
There is a large board in the center of the main waiting area with train schedules and they tend to announce which platform the trains will leave from within minutes of their departure so keep an eye or ear open. It would be a good idea to know the Franklin line schedule before you arrive just to have an idea of how long you must wait for your next train.
Ticket windows and automated ticket kiosks ("charlie ticket kiosks") are a bit cheaper than buying your ticket while on the train, but not too much. It's really up to you.
My question to you is, why take the train all the way into Boston then take it back out to Norwood? Amtrak trains (even the Acela) stop at the Route 128 Station which is either IN Norwood or just across the town line. It would be far easier to just catch a cab to wherever you need to go in Norwood from this station than it would be to go all the way into town then come back out. The Route 128 staion (served by amtrak) is only a few miles from the center of Norwood. It would save a ton of time and cab far wouldn't cost all that much more than a rail ticket from Boston to Norwood.
*Edit*
I love google maps:
Here is how to get from the Route 128 Station by car or cab directly to Norwood Center Station:
http://maps.google.com/maps?daddr=No...=UTF8&t=h&z=13
It's even less mileage if you take University Ave (the street the Amtrak station is on) directly over to Route 1 (South).