Is there such an animal as a RCA analog to USB converter that works?

So here is my problem, self made I admit.
My present preamp is an Exogal Comet Plus.
This has only one analog input which I am using for input from my phono amp.
Now somehow I would like to also run my cassette tape unit through the system but the ONLY spare input left on the Comet is a USB input.
So my question would be if there is such a thing as a RCA analog to USB converter?
Or could I do something like a passive analog preamp before the Comet that would take the inputs from the phono and the tape to run into the single input of the Comet.
Or are all of those options REALLY bad ideas?
And when I want to play cassette I just swap the input leads directly at the back of the Comet?
Thank you