You certainly can create a RaidZ/Mirror/Stripe from any attached drives. But its a very bad idea to use USB drives - both in performance and reliability terms.
There are some other recent threads covering this discussion already.
I think you might have some serious issues since the bus speeds between USB and SATA are so different. I think it's a recipe for problems and data loss. Give it a try and experiment, but I wouldn't trust it with my data.
i decided for:
- Motherboard Intel Desktop Board DG41MJ Mini ITX
- cpu Intel E6600
- case Chieftec BT-02B Mini itx
- 2gb ram max 4gb ddr2
- 2 hdd WD green 1tb
in future: external esata drives