As you can see, our zpool contains a single disk called c4t0d0. Let's add a new disk to our rpool to convert it into mirror. zpool will do it automatically for us. One important point here is that Solaris doesn't support boot devices to be in EFI label format. So, if your new disk label is in EFI format, you need to convert it into SMI label. So, I'll use fdisk command to create a Solaris2 partition and remove any EFI labels if present. My new disk name is c4t1d0.
After creating the partition which covers whole disk area, I copied partition map of original rpool member c4t0d0 to our new disk c4t1d0.
Now, it's time to attach our brand new disk into rpool. After attaching the disk, our rpool pool will be converted into mirror (raid-1). Except one thing, the grub boot manager. We need to install it into new disk by hand using installgrub command.