There is difference in man page of zfs and reality. In examples of manpage zfs if this construction:

       Example 17 Setting sharenfs Property Options on a ZFS File System


       The  following  commands  show  how to set sharenfs property options to
       enable rw access for a set of IP addresses and to  enable  root  access
       for system neo on the tank/home file system.


         # # zfs set sharenfs='rw=@123.123.0.0/16,root=neo' tank/home

But it doesnt work, this construction is working from command line, but gives bad export line in messagess log:

command
zfs set sharenfs=rw=@pokus.starnet.cz,root=0 storage/pokus

Aug 14 12:26:48 storage mountd[1181]: bad exports list line /usr/local/storage/pokus  rw=@pokus.starnet.cz root=0

Fix: 

I found od internet this page:
http://tuxnetworks.blogspot.cz/2011/11/using-zfs-on-freebsd-9.html

and there is working syntax

zfs sharenfs="-maproot=0:0 pokus.starnet.cz" storage/pokus

You can read manpage exports, there is other options which you can use.
How-To-Repeat: Use example from manpage 

zfs set sharenfs=rw=@pokus.starnet.cz,root=0 storage/pokus