I tried to use Dorico with Garritan organ vst. I have created an expression map with, for instance, a ff registration on midi canal 1 and a pp registration on canal 2 (using absoltute midi canal changes). It doesn’t work : the sound doesn’t change when I use the little play arrow in the expression maps editor to test the technique.
Additionnally, there is one thing that I do not understand at all : how trigger my new techniques (let’s say ff and pp in my example) from the write window ? I can see on the right the bundled techniques (pizz etc.) but not the ones I have created in my “Garritan organ” expression map.
Dorico can create custom Playing Techniques that appear in the score and Playback Techniques that tell associate an expression with the VST options for playback. (I know the similar Dorico terms can be confusing.) You need to create both for custom situations and then link the two together so that what you write in the score trigger the appropriate playback technique in the Expression Map.
Thanks Derrek for your answer. May be I’m stupid, but I still not understand : I can see the same window “playback techniques” as you with my own techniques but I don’t see these techniques in write mode, even using the popovers 
You can add/create techniques and make provisions for them in your Expression Maps if your VST soundset supports them.
That’s what I did, but I do not find the created techniques in the Write mode
Additionnally, the Garritan’s vst seems not to respond to midi channel changes in Dorico (while it does in Presonus Notion music)