Anyone ever seen a "wok adapter" for induction cooktops?
I've never seen one of these and I don't think they would work. You could shape a piece of metal to fit the wok base and it would heat up, however you'd run into trouble with the heat transfer. Air is a lousy conductor of heat, you would need direct metal to metal contact - any imperfections in the shape of either the adapter or the wok would reduce heat transfer dramatically. Even a bit of dirt could make a big difference. 

EDIT: responding to the comment about extending the induction effect

Electrical fields follow the inverse square law, so they weaken at a rate determined by the square of the distance from the source. The magnetic field will be significantly less the farther away the metal is, the field at 4cm will be 1/16th of the power then it would be at 1cm, which is why induction pans need as much contact as possible. If the metal of the ring were to extend the magnetic field to the parts of the wok out of contact then that field would not be able to heat it well. But the magnetic field isn't going to get extended as the energy is going to be absorbed by the contact ring.