You may not be able to grow an existing vdev, but you can add another vdev to an existing zpool. And although having all vdevs be identical gives more consistent performance, it’s also possible to mix different vdevs in the same zpool. Multiple vdevs also result in better performance, as each raidz/raidz2/raidz3 vdev only has the equivalent IOPS of a single drive, but without the write hole drawbacks of more traditional RAID5. Recovery time with multiple vdevs would also be much better than a single large vdev, which is becoming more and more of a concern as drive capacities continually increase while IOPS remain more or less the same.