I managed to double check.
So yeah, on Storage Tab, click manage devices for the pool you want to manage.
Then select the device you want to “extend” with another replica (ie another disk to mirror your stripe), then hit extend, then select the drive you wish to use as the additional replica to form the mirror.
“Add VDEV” is not what you want, as that would add another vdev… which would be another stripe in this case.