What is it that makes a multi-fuel different than an old diesel only engine...
The multi-fuel ability comes from a fairly high compression ratio (but not unheard of in straight diesels), a specific and slight change to the combustion chamber (which resides within the piston crown, like many straight diesels), Fuel injectors that do NOT atomize the majority of the fuel charge so it burns from a liquid state (which really doesn't work out so good in a regular diesel engine), and timing settings to accommodate all of the above. Look up or search here for "hypercycle" for more information about it's origins and processes.
The later and more common multi-fuel engines also have the "fuel density compensator" attached to the fuel pump. Since your right foot tells the pump what volume of fuel to inject, different fuels with different energy content yield different horsepower at that given volume. The FDC will (in theory) reset the pump based on the viscosity of the fuel, which among the authorized fuels will compensate such that the same throttle input will result in the same horsepower output regardless of what fuel is used.
As far as converting a regular diesel to multi-fuel... That's a tough one, as there's a lot of stuff to be adapted. Including the fact that "turning up" the multi fuel process to even conservative modern horsepower per displacement standards gets the exhaust awful hot, awful quick. Diesels will run on more than just diesel fuel, just not such a wide variety of fuels. I'd be inclined more to look into the direct or mixable fuels that work out for a conventional diesel engine. Used petroleum lubricating (engine oil, tranny oil, gear oil, etc) is not out of the question. Not straight, but 50/50 is reasonably doable, and higher percentages are possible depending on the weather and your ability to start the engine on straight diesel. The medium and heavy truck folks don't do much more than dispose of oil from an oil change, but pickup truck sites have loads of information on what burns well in what type of injection mechanism and how much you can get away with at what temperatures. I'm sure you could relate some of that information to your engine.