I think you should first figure out how to mount your share correctly inside your main host and be able to write to it using the user you want - then try to make it work from inside a jail, which should not be any different afaik.
When you mount manually from inside the main host, maybe try this way, i have no test system right now but it might help:
Code:
mount_smbfs //FREENAS/SHARENAME /mnt -o username=USERNAME,password='PASSWORD',domain=DOMAIN_IF_ANY,iocharset=utf8,ro
Can you now do a "cd /mnt" and write stuff there as USERNAME ?