Diesel to biodiesel is no big deal.  You probably don't want to switch to 100% biodiesel though because it's hard on the fuel pump and filter.  Mix it 1:10 (biodiesel to diesel).  As for your second question, I'm sure it could be done but it's probably not worth it.  The cost and amount of work to be done wouldn't justify the end result.  Just know that any diesel engine can run on biodiesel.  It will even run on kerosene or WD-40.  I've also seen people add used motor oil to their diesel.  However, if you are going to do this, I would suggest finding some sort of message board or website where people do this on a regular basis.  And remember, anything you do that goes against the manufacturer's recommendations is at your own risk!

The present diesel engine can run on bio-diesel. A small amount of diesel is needed to start the engine. Once at operating temperature the alternate oil will work. Gasoline engines cannot burn diesel but they can burn other fuels like alcohol produced, of course, from plants.