Hi Ted,

The simplest way is to connect your KN7000 to your PC via a USB cable and then use the Technics Audio Recorder program. Load your pre recorded song into the KN. Start recording using Audio recorder. Start your pre recorded song and sing along into your mic connected to the KN. Audio recorder will then produce a recording of the your voice superimposed on the KN output.
Audio Recorder is by no means the best program for the job but it is simple to use and was supplied with your KN. Others including that preferred by Roger are much better. You will need to experiment with levels etc and refine your technique but this should get you started.
The resultant recording will be a .wav file suitable for converting to CD audio or compressed audio such as MP3, WMA etc.
It should be noted that recordings of your voice cannot be saved in Technics format to a floppy or SD card.

Cheers
Len