FreeBSD 9/ZFS: Striped Pool (2 disks) migrating to mirror (onto additional disk)

[O. Hartmann]

I've got a problem. For performance reasons I use two 1TB harddrives as 
a single pool, and as far as I know, ZFS on recent FreeBSD 9/amd64 
treats suchs pools 'striped'. Well, I can measure a performance gain in 
writing and reading. Well, for backup, I use an additional 2TB disk, 
ZFS, compressed, the backup is done manually via rsync (i'm looking for 
more suitable simple backup solutions).
My question is: is it possible to migrate the two-disk pool without data 
loss into a mirrored pool by adding the one 2TB-disk?

Comments appreciated,
thanks in advance,

Oliver