Since the Sony doesn't have any audio outputs you can connect a speaker to line level converter in parallel with the front speakers. Connect that to an audio input on the Kenwood. The Sony volume control will affect the volume of all the speakers but the Kenwood volume control would be able to adjust the remote speakers.
Or get an impedance matching speaker selector and connect both the front and remote speakers to it. No additional
In either case you need to set the Sony to either stereo or multichannel 5.1 stereo mode. This will not allow you to adjust the volume separately though.