During my storage upgrade I ran in the following problem. How to expand an existing zpool in the case that the LVM volume the zpool is refering to grows because of new HDD. The solution is simple:
# old zpool size
zfs list storage
NAME USED AVAIL REFER MOUNTPOINT
storage 3.21T 820G 236K /storage
# to make ZFS aware of the new LV size:
zpool online -e storage /dev/vg0/Logical_Volume