Can you use kerosene in a fuel oil furnace?

Yes and no.

An unmodified oil burner will burn far more kerosene than fuel oil for which it is designed. If you replace the nozzle with a smaller nozzle and adjust the air, using an orsatz [orsat gas analyzer], a device for determining the combustion efficiency of the flame, you will be able to use kerosene.

* interesting fact: Kerosene is also known as #1 fuel oil, or JP5 jet fuel.

Mixing kerosene with #2 fuel oil, (home heating oil) in small amounts, should have no appreciable effect on the furnace efficiency.