Stove Knob adapter 8mm to 6mm?
I couldn't find such an adaptor, but you may be able to make your own. Find a section of small copper pipe with 1mm thick walls, with ID greater than 6MM and OD greater than 8MM. Cut a length to match the D-shaped section of shaft, and then squash it to a tight fit around the shaft. You'll probably need to cut it along the length to reduce some of the circumference; do that on the side opposite the flat of the D (that's where the stress is). Then see if you can get the knob to be a straight fit. It will be iffy, and may not last, as indeed there will be a lot of stress on that flat.