Eugh. Kind of bad news. I thought that RAID10 UFS had been enabled in the GUI under FreeNAS 9 but I don't see it.
So let's experiment. Go to a shell. I have four disks, da3...da6
gpart create -s GPT da3
gpart create -s GPT da4
gpart create -s GPT da5
gpart create -s GPT da6
gmirror label -v -b round-robin mirror1 da3 da4
gmirror label -v -b round-robin mirror2 da5 da6
gstripe label -v stripe /dev/mirror/mirror1 /dev/mirror/mirror2
newfs /dev/stripe/stripe
Go back to GUI, "Auto Import Volume", and import "stripe" (not the mirrorX's!)
This is in no way a promise or guarantee that this will work for you, will continue to work for you, etc. but it seems to work, so feel free to play with it.