I have the same phono converter. It is not possible to play external sources in Roon. The only method is effectively to serve your own IP radio station using Icecast and then use the internet radio station facility in Roon to tune in. Once you got it working it is probably fine but as you have seen it is a lot of work.
As of now, Roon is a closed system. Local files, streamed files and internet radio. Your only other option is to use the NPC to ‘rip’ your vinyl which sort of defeats the point of that format!