I think that is going to be a problem. You need to bring the signal up to line level for your computer (or what ever) to take the signal. One idea that I have is with a digital audio balun, which will (unfortunately) convert it to an unaltered digital signal. Since you are probably going to process this file in your computer anyway, it might be the way to go.
So the path would be turntable>cartridge>digital balun>cat5 wire to some digital connector converter to transform the signal from the cat5 to S/PDIF...........and I'm out on a limb here. You might be able to take the two wires from the cat5 and simply put a RCA connector on it to get it into a converter box (S/PDIF to USB).....
Now I need help....
'ner