Converting a diesel engine to burn propane really doesn't make any sense. But just for argument's sake, let's see what would be involved.

In any internal combustion engine the fuel must start to burn in the cylinder at precisely the right moment. Because of compression ignition, this eliminates introducing gaseous propane into the intake manifold like they do with spark ignition motors. As the piston began to rise, the heat of compression would immediately cause the charge to detonate. There's no way to control when the burn begins.

This implies mechanically injecting liquid propane into the cylinder in the same manner as diesel fuel. Hmm, injecting liquid propane at high pressure: I wonder what kind of pump could do that, especially since the lubricating properties of liquid propane are probably zilch. Once the liquid propane is in the cylinder, it must vaporize. The evaporting propane would absorb a considerable amount of heat and ...

Do you see there's no overriding advantage to such a setup? We spend extra money on diesel engines because it burns a fuel that has a higher heat content per cc, is easier to refine, and costs less (at least it's supposed to!). That translates to savings. Figuring out how to do it with propane as a fuel is a lot of pain for no gain compared to a spark ignition motor.