There is a longstanding feature request [1], as yet unimplemented, for Roon to provide an audio output device so that it could accept, process, and route arbitrary audio like streams and digitized audio from a turntable. I think it would be a useful feature–it would seem to address your turntable and many other potential sources–but it appears not be a priority at this time.
[1] Roon as an output channel for other sources? Ugh, but maybe
See also Line-in input as source, where @brian says this is “something we would really like to do at some point.”