It is POSSIBLE to CNC a manual lathe without losing any manual capability. You use a ball screw on the cross slide but use the existing carriage drive (not the lead screw) to move the carriage. You control the backlash with an air spring, control the accuracy with linear scales on the axes. CNC threading is no problem, just requires an encoder on the spindle. I designed such a system for my 13x40 but never followed through on it because I bought a CNC lathe.