FreeNAS - Adding an additional drive to a striped volume

I have recently started using freenas and have two 250GB drives striped (for now) and want to add another drive to the mix. How do I go about doing this? I've read the docs, they mention extending ZFS pools (which I think I need to look into) but not standard volumes..
I'm new to all this and should probably not even be trying it as I'm using an old dell tower, sticking in 8GB of memory and using extended cables outside to the drive bay from my bitfenix prodigy.
Anyway what should I be doing? Should I backup my data and start again, making a ZFS pool and use raidz?
My current setup:

I don't think you can add a drive to the striped drives you have now without wiping the data and creating a new drive pool, so yeah back up your data and give it a go. As to whether you should even be trying this with the hardware your else will you learn?

I think I played with several different configurations (used boxes and drives) before I spent the money to invest in the hardware I needed for my NAS, if I hadn't learned on junk hardware it would have taken me a very long time to figure out what I wanted and needed....just go for it as long as your data is backed up you have nothing but your time learning to loose which I would consider a investment.

ZFS does striping across all the VDEVs in a pool. FreeNAS uses terminology carried over from days before ZFS, so it can be a bit confusing, but essentially a “Volume” is the equivalent of a pool. So you can easily extend a volume by adding an additional disk.

(In case people are still finding this, here is a more up to date link:
The new UI has moved away from the old terminology and toward a more consistent use of ZFS terms, so it’s a lot less confusing now.)

thanks, this helped me a whole lot, 6 years old and still useful.