I believe you have a couple of options.

1- if you are on a PC then input you song into RealBand and regenerate those measures that do not harmonize correctly. You may have to do this a few times. Note that I do not use RealBand so someone else will have to fill in the details on how to do this.

2- transfer you song to a DAW and use MIDI to write your Cello part. There are a number of Cello VSTis available ranging from free to very expensive. Google/Bing cello vst and you will find them.

Good luck.

Whenever I get something stuck in the back of my throat, I dislodge it by drinking a beer.
It's called the Heineken Maneuver.

64 bit Win 10 Pro, the latest BiaB/RB, Roland Octa-Capture audio interface, a ton of software/hardware