Since Rigol also sells a DS1104Z and this is more expensive, I cannot imagine that this option can ever be made legal by a dealer.
Well, Rigol does sell official bandwidth upgrades for other scope series, e.g. the MSO5000. Any combination from 70/100/200 MHz to 100/200/350 MHz is offered. Of course they will cost you, a bit more that the price difference between the respective models; you can check the pricing structure e.g. at batronix.com.
But for whatever marketing reasons, Rigol have so far decided against this for the DS1000Z series.