Well, it is possible, but usually not worth the trouble. There's an import option to get data off random volumes with a number of popular filesystems. Check the manual instead of random websites.
Connect the USB device to your laptop/PC and rsync or robocopy the files from FreeNAS to the USB drive.
Two peneth of advice. To speed things up (big increase in my case) use USB3 external device if you can and Ethernet, don't use wireless and don't use Powerline.