Quite a few have done CNC lathe conversions. Generally the lead screws are replaced with ball screws, so most can't be used manually anymore. Biggest issue with ball screw replacement is that the ball nuts are usually quite a bit larger than acme nuts, so are hard to fit. Most avoid just throwing steppers on the existing screws because the backlash in most of them is too much for CNC.
I don't think you'd be happy with the surface finish you'd get if you added a CNC stage like that ebay one to your lathe or mill. Nowhere near the rigidity you're used to.