That's a bad idea. Conditionally.
In order to support multiple access to a block storage device (SAN, like iSCSI) you need a cluster-aware filesystem (like VMware's vmfs). Your typical crummy PC filesystem isn't aware of the possibility that someone else could be frobbing around with the blocks on the disk, and will cache things "helpfully" to make it all faster, and will then freak when things on disk are inconsistent with what the data in cache told it to expect. Writing is even worse.
Your choices are to use FreeNAS as a NAS (and not as a SAN) by using CIFS or some file sharing protocol, which means that FreeNAS will happily do all the sharing work, and you can get the benefits of ZFS and all that. Or you can use FreeNAS as a SAN by using iSCSI, then you will want to mount the iSCSI drive on your WMC and have WMC do the sharing.
I'm kind of curious where you would get the idea to use iSCSI for this.