We welded one of the top rollers on our 605C a number of years ago. It is a tedious process, lots of grinding.
You have to do everything with the roller in the chamber in order to keep everything tight and inline. You can do what cfpinz said and put a similar pipe in the center, although I would encourage you to do the full length of pipe for the roller, not just a section...our you will design yourself a weak spot. Regardless, in that scenario, you would still have to torch out the stub shafts at the ends to get the pipe into the roller.
Your lucky. Vermeer 605C rollers are priced at $1000.00 a piece. In your case, after the 2nd roller breaking, I would go find another baler.
Also I should add, welding is not a sure thing. Might break in the same spot once or twice more.
Tip: don't weld the two pieces in the center (the crack). Drizzle the weld back and forth over the crack. After this is done, then weld the center. This will allow your roller to flex and avoid breaking again. :cowboy: