We can export an virtual volume (zvol) on TrueNas, thanks to the iSCSI protocol.
Storage
Create a pool
Create a zvol from the pool
Sharing / Block/Shars (iscsi)
Target Global Configuration: Modify iqn if necessary
Portals: Add a portal with IP 0.0.0.0
Initiators Groups: Add a initiator with feature of “Allow All initiators”
Authorized Access: No modification
Targets: Add a target with the above portal id, initiator id
Extents: Add a extent with created pool/zvol
Associated Targets: Add an associated target with created target and extent
Services
Launch iscsi service, and start it automaticlly