There's a number of possibilities. I offer this as information gleaned from the web - I've done no research myself, so I may be flat-out wrong.
In the last year or two, Ford has offerred an F-450 pickup. Presumably, one could take that same bed and put it on an F-550.
What you can't do is "simply" take a standard Ford pickup dually bed and stick it on an F-450 or F-550.
A cab/chassis is a different length (by about 4") from the rear of the cab to the wheels, so to get a standard bed to fit, you have to (well, would want to) add a spacer between the cab and bed. This can also, of course, be done with some body-work.
I'm actually not sure what Ford did with their bed - it it longer than 8', or did they actually change the wheelbase.
Also, a pickup chassis has "curcy" frame rails over the rear wheels. A cab/chassis has straight rails. This is certainly possible to overcome, but you don't "just" bolt them on.
I also heard something about, for some future model year (or maybe it's current now?), Ford having a light-duty F-450, that's really a F-350 with a few beefed-up parts, and available as only a pickup. Then, they'd drop the F-450 pickup option.
The whole thing is abour appealing to 5th wheelers, who want a pickup, but want a bigger truck. It's not a huge market segment, but it's just big enough so Ford keeps looking at it.