Recently i had to add new drive but it doesn't had any glable.
Since drives are same size best bet is to use same partition size for new drive.
Using gpart backup / restore one add new partition on new disk and good to go for zfs.

gpart backup da11 > da11.bkup
gpart restore -l da34 < da11.bkup
glabel status
gpart show /dev/da34
gpart show /dev/da11
 
zpool add nas spare gptid/<New lable>