Turns out to be relatively easy if you use the Docker and Portainer Aps in ADM and then pull the image of the Extension Manager over - the available extensions can then be selected.
Wow, this was a while ago so can’t really remember - I ended up putting a Raspberri Pi on my Rotel kit as it predated streaming and I never got a Rotel Preamp or Processor in the end (at least not yet and they are now selling full roon ready goodies).
From memory, there is a developer on these boards who had the instructions published in a chat string, just search for them and it should be out there.
I also pulled my roon core off the Asustor and got a NUC and use Roon Rock on it - difference in performance was night and day.