I think you misunderstood winnielinnie. He meant to just connect the scanner to your desktop pc/laptop, have truenas connected with an smb network share to that machine and if you scan a document just save it to that network share and not your local drive.
Some scanners that have a network port can be configured to directly scan to smb shares over your network.
But i don't think it's possible to connect the scanner directly to your truenasbox and make truenas see the scanner and configure it via the gui.