1. Yes, you can put a complete cab, but you will have to script it of course.
2. By custom you mean your own model? If so, pretty easy, you get your bed, then you import it inside the truck i3d file, you move it to the position you want it to be. Then you copy the attacherJointBody i think it is called, you paste it again inside the truck main component, then you select it and with the middle mouse button you drag it inside the bed main component. After that you can export the bed and finish it as a a separate mod. You can also do the opposite, where you imprt the truck inside the bed i3d and do the same process, move it, and this time you can just select the attacherJointBody and directly drag it into the bed main component.