How about a Stereo MiniPlug to Left and Right RCA adapter like this:

www.amazon.com/.../B00004Z5CP

And feed it through the on-board line in to the computer?

I have a mono miniplug version, which works fine for 78's - but never got around
to doing my vinyl. I avoided the USB player route, as the decks are pretty
basic - but I guess if you are converting to CD that isn't a problem? I'm
assuming the standard line-in is stereo, of course (I may be mistaken).