Depending on how you have the arrays organized, I'm sure you can expand a zpool with additional sets.

http://www.itsacon.net/computers/unix/expanding-a-zfs-pool/

zpool add -f poolname raidz[or raidz1,raidz2,etc] /dev/driveid /dev/driveid /dev/driveid ...

You'd then have a pool with another raidz section within it, whose size was represented by the capacity of each of the sections within.