Expanding a raidz (vdev) is a zfs limitation, cannot be solved by freenas itself.
Raid 10 is possible by adding a new mirror to an already mirrored volume (by specifying the same volume name in the volume creator)
raidz2 is a raid6 without the write hole..
a set of several mirrors is a raid 10.
zfs allows any arbitrary collection of disks to be added to an existing pool. you can have a raidz2 of 6 disks and then add 2 disks in a mirror 6 months later. we just need the web gui to support it. hope that comes in 8.1 :)
that being said, actually expanding the array is a limitation of ZFS and not of FreeNAS.