I actually asked this in the #freenas IRC channel, and they were able to answer my question.
To answer my own question: no, you cannot add an additional drive to a RAID-Z1 array to make it into a RAID-Z2 array. The nor can you add additional drives to scale the array horizontally. The geometry of the array is set when you first build it.
You can however, grow the capacity vertically by adding larger drives one by one. So, I could grow my 6x4TB RAID-Z2 array by replacing each drive with an 8TB drive, and allowing it to rebuild each time I swap out a drive. The details of that are beyond me though.
They also strongly recommened against setting up a RAID-Z1 array. Always use Z2 or better to avoid losing the whole array if you lose two drives.