You can apparently add a new raidz, though. So adding 3 new drives would be possible (but not very efficient):

http://alp-notes.blogspot.no/2011/09/adding-vdev-to-raidz-po...

[edit: perhaps there should be a listing of "replies added since you hit reply"-function (with a "confirm" to reply anyway-button) to cut down on redundant replies.. and trade them for upvoting reasonable replies...]