My solution has been to carry an extra throttle grip in my panniers. I have a BrakeAway throttle lock so the end of the replacement grip is turned down so it will still fit my throttle lock.
I have been thinking about buying some Avon Contoured grips, but in answer to your question....according to the instructions I read for the Avon grips, the piece you are talking about has to be glued to the grip with the adhesive provided. I don't believe you can simply let it set on the teeth of the grip that it mates or locks into. If the piece breaks out, it would be tough to replace it with it glued on. If someone on the forum has not glued theirs on, it would be interesting to know how that has worked out for them.