If the NFS mount is visible in the jail's namespace, then the jailed processes can access it subject to normal access control restrictions. However, processes in jail are not permitted to mount, remount, or unmount filesystems, so any access to NFS must be configured by a process outside the jail (and preferably, before any untrusted processes run in the jail, so as to prevent racing and path-based games). Typically, when using NFS with a jail, I'll do the NFS mounting prior to actually starting the jail.