I asked this question elsewhere and the consensus was it should be doable, though there was some argument about the details. But I figure the more people who weighs in on it, the better.
I got a 20mm arbor on my table saw, the standard in Europe where I am is 30mm and I wouldn't mind having to not bother with adapter rings, though my main idea is also make a longer arbor so I could fit a dado stack. I heard somewhere about someone making an adapter sleeve for his table saw and that's where the idea took root.
What I was thinking was an internally threaded part that simply screws over the arbor I currently got. Arbor pictured below:
Someone suggested the adapter being held in place with a grub screw and another said it was not required because the thread pitch means it's self tightening and a grub screw would prevent it locking down tight. Any opinions on this here, sound doable or just plain crazy? I have no idea what it'd cost to manufacture such a part either, if it gets too expensive I think I'm better off getting a router (something I plan on eventually anyway). I might also build Matthias Wandels screw advance box joint jig to get around this lack of dado capacity for now.