I used the AA and did exactly this Evel.
I spend 8 years on a motorbike from 16 onwards. I asked them if they would be able to transfer some of my bike NCD over to a car. They managed to honor 4 years of the total 8 I had built up. Obviously I didn't get the full whack but the money it saved more than made up for the 4 years I tecnically lost.
Some companies don't mind others seem to freak out about it. I decided to stay with the AA at the time. But my guess is most will not honour it.
I beleive I've tecnhcially lost all my motorbike NCD now I have had a car for such a long time.
Alfa 166 2.0 Twinnie - LPG and a few trick performance parts