You cannot convert a fuel oil burner into a waste oil burner for safety purposes. If you were to heat waste oil in a standard burner, the impurities in the oil would clog the sprayer. These impurities are filtered off in a waste oil burner, but in a fuel oil burner they would be trapped in the sprayer or released into the air.
Waste oil needs to be heated to a higher temperature in order to imitate the properties of fuel oil, whereas fuel oil can be burnt at a lower temperature. If you were to put waste oil into a fuel oil burner it would not heat the oil to the right temperature, and burning would not be effective.