As of yet, there isn't a way to convert zpool structures. Also, there isn't a way to expand a RAIDz. To my knowledge, RAIDz is something you have to setup from the start.
That said, there is an exception. If you have three disks in a RAIDz configuration, basically one disk is used for redundancy. You can concatenate zpools so you can create a second three disk RAIDz and that the two work together. That way each three disk RAIDz is fault tolerant within itself. The down side is not you have to use two drive for fault tolerance where as if you build the RAIDz with six drives from the start you would only be required to use one.
There is a second exception. (I've only done this in VMware to test it.) If you have a RAIDz zpool, and want to increase the capacity, you can swap you the drives one by one with a larger capacity drive. Then after the last drive has rebuilt, (I think) you can export the pool, and import it back and ZFS will see the new capacity on the drive and begin to use it. I read this off a blog I can't locate and it was a while ago, so there may be additional steps.
Some people have considered using the copies property of ZFS to spread extra copies across a stripe zpool. Here is a site that talks about the copies property. ZFS will attempt to put the two copies on two different drives, but it doesn't have to. So the data may be fault tolerant, or may not.
I'm hoping the FreeNAS because it is built on a flavor of BSD will get the latest bits soon. OpenIndiana had the latest versions of ZFS incorporated (zpool version 28 and zfs version 5). Also, I've read ZFS has been ported to Linux (not just with FUSE).
I used to use FreeNAS because it was easy to setup. The I moved around to various OSs chasing the latest versions of ZFS mainly because I wanted the dedupe feature to extend the capacity of my storage.
I know when ZFS get in place migration between zpool types and dynamic expansion of RAIDz many ZFS people will be happy.
I have tested this with ZFS and write performance is about half what it should be, because ZFS distributes reads and writes over all vdevs (therefore dividing I/O to several places on the same disk). Thus, the speed is limited by the speed of the disk with the most partitions. Read speed seems to be equal to the disk bandwidth. Note a pair of ZFS partitions on two disks has roughly double the read speed of either single disk, because it can read from the disks in parallel.
Using MD LINEAR arrays or LVM to create the two halves results in twice the write performance compared to the above ZFS proposal, but has the disadvantage that LVM and MD have no idea where the data is stored. In the event of a disk failure or upgrade, one side of the array must be entirely destroyed and resynced/reslivered, followed by the other side. (e.g. the resync/resliver has to copy 2*(size of array))
Therefore it seems then that the optimal solution is to create a single ZFS mirror vdev across two LVM or MD LINEAR devices which combine the disks into equal-sized "halves". This has roughly twice the read bandwidth of any one disk, and write bandwidth is equal to the individual disk bandwidths.
Using BTRFS raid1 instead of ZFS also works, but has half the read bandwidth because ZFS distributes its reads to double the bandwidth, while it appears BTRFS does not (according to my tests). BTRFS has the advantage that partitions can be shrunk, while they cannot with ZFS (so if after a failure you have lots of empty space, with BTRFS it's possible to rebuild a smaller redundant array by shrinking the filesystem, then rearranging the disks).
This is tedious to do by hand but easy with some good scripts.
Best Answer
It is the same as with other vdev types. The FreeNAS documentation explains how it is done, see figure and table 8.1a for details. Note that only available disks will be shown and they limit the possible configurations, so make sure that all your disks are properly recognized.
If it does not work this way, you could also create a striped pool out of two single disk vdevs and then immediately attach a disk to each vdev (resilvering will be nearly instantaneous).
You can also do it in the shell, but I would try with the GUI first.