Thanks to researchers at the UNSW Engine Research Laboratory, it seems a dual-fuel diesel-hydrogen engine is possible.
At the moment, if you want to convert your diesel fleet to hydrogen, you need to swap out the entire engine with a hydrogen fuel cell option.
Do-able, but more expensive than keeping the original diesel injection engine but adding a hydrogen fuel injection unit. With stratification the blend produces less nitrogen-oxide emissions than normal diesel, about 85% less carbon and operates at about 26% greater efficiency.
https://hydrogen-central.com/new-system-retrofits-diesel-engines-run-90-per-cent-hydrogen/