This is an incredible friendly place for all Ketron users.
https://www.facebook.com/groups/KetronSD90/?ref=share
I asked a question there and one of the other users made a video for me to explain.
Its a great place to join for every Ketron user.

Fiinally after 2 years of asking this question i got my answer.
Yes it is possible to change GM voices in a styles.
And its actually quite easy..
And you can change the changes in a) the style b)as a user voice c) overwrite the orriginal voice. This is a link to the video he made..

https://drive.google.com/file/d/1ZFjBRx3wYVoMQ_xr-8SEzcr0GnxoCzYY/view?usp=sharing

Now i am back to seriously considering the SD90 again.