If you're playing v1.1 of Village Sim (which was released in late 2005), you can always train builders by having them fix existing huts. Just drag a villager to one of the 3 huts that they built, and they will gain construction skill each time they successfully repair a hut.

If you're playing v1.0 of Village Sim, the only opportunity to train a builder comes from an Island Event - a storm that washes more debris onto the beach.