By converter are you looking for a filtration system, an aseembly to run veggie oil as a fuel, or a biodiesel refinery?
The filtering can be done onboard although I will tell you from experience that it likely isn't going to be real fast, smells a little, and takes up a lot of space.
If you're looking to run veggie oil as a fuel, just look around. There are a bunch of people running a system on their bus, myself included. Jason (lapeer20m) probably has the most well documented system.
If you're looking to put a biodiesel plant in the back of the bus...well...I would urge you to reconsider. You will still be plagued by the filtering issues, but more importantly, you'd be hauling around lye and methanol, both very dangerous. In fact, I'm not sure what the legality of such would be as both are likely classified as hazardous materials.