If I remember your frame correctly, I would suggest you first try and build the kit frame instead of using your old frame. It doesn't take very long to assemble it. Unless you want your mecanum drive to compete in an event, I would skip using the wheel brackets and just mount the wheels to axles going through the c-channel frame. This will get you a simple and rigid platform for testing and stuff.
From there, converting to a mecanum drive would require for you guys, two more gearboxes, two more CIM motors, and a bit of modification. Move the gearboxes you have a bit closer to the wheels, and mount two more basically the same way as the other two were. From there you should be able to get rolling and you'll have a base your programmers can use.
I think if you guys plan to compete with the bot at an off season, a better investment would be in two more CIM motors in your existing gearboxes (this increases your acceleration and your ability to push other robots) and some omni wheels. The omni wheels will allow your robot to turn a lot better than it did at CT, and you won't have to reprogram your whole robot. It's also a LOT cheaper than a mecanum base, though way less cool I know
(I would personally not advise using a mecanum drive in a competition robot)