Just add another VDev. For example lets say you have 32 drives. It makes more sense to structure it as 4 x 8drive raidz2 instances rather than 2 x 16drive raidz3. Because then you will only have to add 8 drives vs 16 to expand the pool. You could incrementally expand your ZPool by adding new vdevs of the same or larger size. You could technically make your vdevs 4 drives in raidz1 but id recommend at least 6 drives per vdev in raidz2 or greater