Can you mix kerosene with home heating oil without damage to furnace or contaminating home heating oil?
Yes you can; According to "FlashOffRoad"
"Kerosene is routinely added to home heating oil, in large quantities. The furnace doesn't know, or care. The furnace oil pump does not have the same clearances (they are more crude, greater clearances, lower pressure...) and the kerosene won't hurt them. Most will (and often do) run on straight kerosene--here in NH, if the oil tank is outside, the mix will be either 50/50 or straight kerosene. Kerosene doesn't have the same heat values either, you won't get the same amount of power from a gallon of kerosene as from heating oil, or diesel fuel."
See full article for more detail <http://flashoffroad.com/Diesel/DieselFuel/about_diesel_fuel.htm>
Personally though, I wouldn't add more than 10 gallons per 275 tank full just to be safe.