Extend 1 Harddrive to existing 2Harddrive using Stripe (raid 0)

[nandaanubis]

I want ask ., its possible in freenas using 3 harddrive stripe .,. im have issue ..i hv 1 pool existing with 2 drive mode stripe ., then i want extend it with 1 drive again .,
But i cant add it..error showing like this
You are trying to add a virtual device consisting of 1 device(s) in a pool that has a virtual device consisting of 2 device(s)