[Taylor]

You're going to need two more gearboxes and two more motors. In order for a mecanum drive to do all it can, each wheel needs to be independently driven. The way the robot moves depends on how the wheels work with/against each other. There are many great white papers in the CD-Media section; search "mecanum" and you'll find some nice resources.
Mechanically, your chassis will be the same. Simply place the gearboxes directly above the wheel pods, and run a chain from the gearbox sprocket to the wheel sprocket.
Good luck!