So I can leave the stereo busses as they are and create new 5.1 ones next to them ?
Yes. After routing them to the 5.1 you can pan the stereo signal to any position in the 5.1. For example to left - right, or to front, or to left surround - right surround…or you can use a upmix plugIn, that will spread the sound to all 6 channels (or to some of them)
Oswald