This is not possible, and does not make sense, what if there is another subclass of AbstractNestedType that does not have nestedField2 ?
What you can do is to use
.customize(new CustomMapper<> {
void mapBToA(FakeBeanA a, FakeBeanB b) {
if(b.fieldB instanceof NestedType) {
a.fieldA2 = ((NestedType)b.fieldB).nestedField2;
}
}});
...