This can be done using MuseScore as an intermediary step. In MuseScore you can go into a stave properties and tell it to transpose the guitar (or any instrument). So, for example, if you wish to capo at fret three, tell MuseScore to transpose the instrument up by a minor third.
If you then export this as a musicxml file and import it into Dorico then Dorico respects the transposition. If the instrument was defined as a guitar in MuseScore, then Dorico will allow you to add a tab stave which will also work properly.
All Dorico needs is the same ability to transpose any instrument individually.
One thing that I’ve found though is that it is best to prepare the whole piece before using MuseScore to do this. You cannot simply import the guitar line from MuseScore. Dorico will mess it up if you change they key elsewhere.
I write songs, so I generally only need the voice and guitar. If I was also adding a piano part I would do it all in either MuseScore or Dorico before passing it through Musescore.
Chord symbols and text such as titles don’t seem to pass through this process, so you need to tackle it methodically. What I might do is prepare the score completely, then reimport it in a new score and copy across anything that went missing.
One thing here that annoys me about Dorico is that it seems to be impossible to select everything and filter our just the chords. But I’m probably missing something.
One more thing: Dorico will show that you have a guitar in Eb if you follow the above. This will confuse every guitarist on the planet, so remember to go into edit instrument name and turn off the ‘in Eb’ with the option below the edit boxes.