I see two problems. One is how to get the knob to attach to the keyed shaft. The second is how to get it to also attach to the non-keyed shaft.

I don't know how the current knob is attached to the keyed shaft, but it's attached to the non-keyed shaft by means of an O-ring, as I've mentioned before. More specifically, one puts this O-ring into the knob, where it just barely fits. There is a shelf inside the knob's hole on which it rests, but the shaft extends past it into a smaller hole that keeps it level, while the O-ring is sized just right so that it presses against both the knob and shaft, keeping it from moving in and out without significant effort. I don't see why the same system can't be used for the new knob. You'd need to make sure the shelf was rounded so that the shaft wouldn't catch on it when applying the knob, but little beyond that need be changed from the plastic version.

I haven't seen a keyed shaft, but I assume that it's not keyed along its entire length. There ought to be a point at which it becomes a solid cylinder again. So why not make a patch to fill out the key so that it becomes a full cylinder again? It could conceivably be made out of aluminum as well, but it might make more sense to make it out of hard rubber or plastic instead. It might be a little tricky holding the patch while pushing the knob on, but, once the knob's applied, the same pressure holding it on should hold the patch on as well.

This all assumes that the keyed shaft is of the same diameter as the non-keyed shaft, but an O-ring with a different internal diameter should be able to fix that problem as well. In addition, the O-rings and small patches should be cheap, at least in comparison to the knob itself.