I haven't seen one yet.

You can find information on getting ARIA on the whitelist here:

https://www.steinberg.net/nc/en/support/knowledgebase_new/show_details/kb_show/how-to-use-vst2-plug-ins-in-dorico.html

and more information/discussions, including exactly what to add to the whitelist for ARIA can be found here:

https://www.steinberg.net/forums/viewtopic.php?f=246&t=104927

Once you've got some ARIA instances hosted, and staves assigned to use loaded ARIA instrument slots (You'll need to set them up manually in the ARIA Interface), you'll load in the expression maps and begin the process of learning how Dorico can make use of them during playback.  The Dorico forums should be helpful for researching or asking specific questions that aren't clear in the Dorico documentation, or turn out to be less than intuitive to figure out on your own.

How many expression maps are there?

Some Garritan Libraries come with Cubase expression maps (which can also be loaded into Dorico).  GPO4 came with about 10 of them that can be found in your Personal Orchestra 4 installation directory.  They mostly just supply key switches for bow style changes in string instruments.  Some of the instruments that offered 'articulation' key switches like staccato or marcato simply assign a key-switch to notes 'marked staccato' in the score.  Much more is possible with such a tool though (and you'll typically want different interpretations for different passages and scores, thus a mostly blank template)...

These maps are also valid for  the legacy content in GPO5 (Strings that were NOT part of the new "Garritan Orchestral Strings" collection that were added in version 5).

As far as I know, GPO5 did not ship with any of these expressionmap files in the installation, but you can still find and download them somewhere on the Garritan web site (and other support sites).

Note, the few maps that were included in GPO4 are pretty simple in nature.  They were basically designed to give the Cubase Score and Key editors easy access to key switches.  I.E.  Swapping between acro and pizzicato.  Users would add other things as needed in a score (I.E. Send a whole slate of CC messages to ARIA based on Score symbols or text entries).

Such maps are really simple to make and edit yourself as needed.

(Image showing the expressionmaps that shipped with Garritan Personal Orchestra 4)

I've got to go to work soon, but later tonight I'll see if I can't put together some example screen shots to demonstrate some of this working inside Dorico for you.

For the most part, you build things as you need it and can recycle those efforts in all of your projects.  It would be a monumental task for anyone to try to predict and supply all the possible 'score interpretations' that a composer might like, and even if someone did attempt this, it would be more complicated to document and explain than it is to just give the composer the tools to 'make his own score interpretation' on the fly.  Don't stress though, it's not very difficult once you see how the tools can be applied.

Thank you very much

Sorry it took me so long to get back to this.  Follow this thread to see what I've put together thus far.  It's not all that comprehensive, but it's a start, and will hopefully get the wheels going in your mind on how all this can work together:

https://www.steinberg.net/forums/viewtopic.php?f=246&t=113796&p=622003#p622003

I've created a 'starting' set of Dorico Expression Maps for GPO5, along with project files for GOS/GPO Strings, SAM/GPO Brass, etc, so that you can save Endpoints easily and then create your own Playback Templates. There's also an instruction PDF, which is well worth a read and a spreadsheet of keyswitches. 

Because GPO5 offers such variety - so many different instruments - each person's uses and needs are likely to be very different. But hopefully this is useful as a starting point for further tinkering. 

https://www.steinberg.net/forums/viewtopic.php?f=246&t=201477

I haven't done any Percussion yet. I'm more intrigued by the prospect of setting up the Custom Organ Console, so that you can actually trigger stops with registration instructions in the score. But that's 'some' work. 

(Naturally, many thanks to Brian for various posts on various forums.)