Dually Question

First, thank you to StealThee and 1965Comet for the comments on the ride of the F-550. Took one for a test drive, even one set up for towing a fifth wheel and immediately set an appointment for my chiropractor. Holy Smokes you two weren't kidding rough ride.

So I am looking at F-350's. One thing I was told to try and get was a dually, seems common sense that having four tires in back is better than two for several reasons.

Again, showing my lack of truck knowledge here. If I find a F-350 HD that is really nice but only has single wheels in back can you convert it to a dually?

All the ones I am looking at are flatbeds so fenders aren't a concern. Is it expensive?

Or should I just skip the single wheeled ones and get a dually in the first place?

Thanks again for any help

Here's a link to another forum where they discussed this issue.
https://www.powerstroke.org/forum/suspension-steering/171546-converting-single-wheel-dually.html