I have one of the Bodnar pedal cables. The plug where you plug the pedals in contains a small microcontroller, which exposes the pedals as a USB HID device (as well as interprets the pin out from the pedals into something useful. The shifter would do the same as well.) The shifter cable has some configuration as well, to attempt to work around the inevitable breakage of the Logitech shifters.
You cannot simply splice a serial cable to a USB cable. USB Cable has 2 data pins (+ and -), where the pedals alone would have at least 4 pins (Signal for each, plus a shared ground). Never mind that the signalling is incompatible.