Hi AJ,
I'm not familiar with the Casio, but all the modern vocal harmonizers seem to have a direct 1/4" input that triggers harmony. Electro Harmoniz makes pedals that track guitars into synth/piano/organ sounds too.
https://www.sweetwater.com/store/detail/MEL9--electro-harmonix-mel9-tape-replay-machine-pedal
https://www.sweetwater.com/store/detail/Synth9--electro-harmonix-synth9-synthesizer-machine


Just a thought ... since the technology exists.