zpool attach [-f] pool device new_device
Attaches new_device to an existing zpool device. The existing device
cannot be part of a raidz configuration. If device is not currently
part of a mirrored configuration, device automatically transforms
into a two-way mirror of device and new_device. If device is part of
a two-way mirror, attaching new_device creates a three-way mirror,
and so on. In either case, new_device begins to resilver immediately.
-f Forces use of new_device, even if its appears to be in use.
Not all devices can be overridden in this manner.