In general, FreeNAS is designed to work as a file server, not as a file server
client. It'll share its own files all day long, but nothing in its UI is designed to let you connect with shares from other servers. It can be done through the CLI, but it takes some doing, and the changes you make may not survive upgrades.
This is a bit longer way of saying that, while what you're asking to do is probably possible, the method that
@HeloJunkie suggests is probably a better way to go.