I’m also intrigued about this but I haven’t dug into it further than the R part of R&D.
You would think that USB analogue to digital converters like the ones used to connect instruments to computers would be able to be used as the source.
A different take is to turn your turntable into a shoutcast stream and let volumio consume it as though it is an Internet radio station. Here is a discussion on that. Is this the easiest way to connect an analogue turntable?
Cheers,
Andrew