Or are you thinking of a live situation, rather than recording?
You could mike up the cello and feed it in for processing. An electric pick up will have a different output level and impedance to a mike so that might not be successful. Is there such a thing as an internal mike for a cello, like the ones you can get for an acoustic guitar? Never looked for one, so I don't know!
The only trouble with miking, processing and amplifying through the keyboard's speakers is that you might get feedback from the speakers into the cello's mike. Needs to be close, and directional.