Hmm - Maybe I don't understand what you said, but I'm pretty sure that I can change the voices used in a Ketron style. Can you give an example of what you want to change and we will attempt to re-create here on our SD90. Matter of fact this would make an excellent topic for the "How-To" series we are working on.
Try changing the envelope or the filter cutoff of the base sound in a style..
And then save it..