Just a push connection, usually just a press fit, possibly epoxied/glued fit from the factory. Chevy uses a similar design on theirs. I haven't ever seen an adapter to use a screw on style knob, but I've used shift knobs that you buy from Autozone and similar that use a sleeve and then a set screw. The sleeves weren't small enough if I'm remembering correctly though. You could always make something wacky and custom like I did with one of my old 1969 C10s a while back. The billiard ball was just drilled out slightly larger than the shaft diameter and I used a piece of rubber bicycle tube around the shaft to help hold it solid. Didn't like the huge size of the ball, so I switched to a Captain Morgan souvenir bottle and used Gorilla Glue to hold it on as I thought the color resembled rum.
