ESXi can use shared storage for a datastore, either as a FC/iSCSI blockdevice or via NFS.
I would prefer NFS for simplicity.
Create a filesystem in napp-it and set NFS to on.
In ESXi storage settings, add the NFS share as a datastore
see
http://www.napp-it.org/doc/downloads/napp-in-one.pdf
A proven ultrastable combination: ESXi 5.5U2 + OmniOS 151012
Esxi 5.5 and 5.5U1 have bugs, the first with e1000, the second with NFS
With the very new ESXi 6.0 and OmniOS 151014, there are some NFS
problem reports at the OmniOS mailing list with an unclear state.
If you want to use ZFS snapshots + replication for ESXi backup,
include ESXi hot memory snaps in ZFS snaps (see menu Jobs >> ESXi hot snaps)