When you say a set.. does it include the mounting hardware? Or is it just a 'matching' set of rear wheels/tires?
It could be done with either the lug bolts, or the wheel weight holes. I would guess the wheel weight holes are easier, especially if it's a DIY setup.
What are you hoping to get from the duals, though? Splitting your ground pressure between more tires is generally not going to help you other than in mud. The only way i would run duals on these things is if i had a worn set of outer tires and a new set of inner tires, so that usually only the inner tires were providing traction, and the outer wheels were mostly serving as some very large wheel weights, while also making the tractor slightly harder to tip over, and less likely to get stuck in mud. But if you have true matching duals and are running on hard surfaces i would not expect it to increase traction.
But it might look cool and im just fine with that!