The issue I see with your suggestion is that after a few hundred gallons of bio-diesel you will have spent the same as you would just buying the conversion. At least with the conversion you can burn waste vegetable oil with costs nothing as long as you can find a restaurant or two that will let you take their used oil. Just got to find the good stuff. Usually a Chinese restaurant will have it since their flash frying method requires the oil be pretty good and not broken down.
This is CABL.com posting #166837. Tiny Link: cabl.co/mRy5