Onan,
'tain't that simple. Once you have pax in the cabin it downgrades from a class E cargo compartment and that means there's all sorts of things you can't carry.
What you're thinking about is a QC aircraft where they roll pallets of cargo out and then roll seat pallets in so the aircraft goes from full cargo to full pax.
The other occasion is when carrying horses on a full charter. There's normally a pallet of seats at the back for the grooms and you don't worry about the down grade of the cargo compartment if you're only carrying horses.
With the latest security legislation, if you're going to carry pax in any form, other than certain company employees, on a freighter, then you need a reinforced cockpit door and that costs money!
Hope that's clarified it for you.
RT