You are going in the wrong direction in multiple ways.
TrueNAS is not designed to be a client and mount storage; it’s designed to own and serve storage.
TrueNAS only uses ZFS. No ext3/4 or NTFS from your old NAS.
ZFS does not work well with unreliable connections, such as USB.
If you were to wipe the old NAS box and set up a pool in it, what’s in the box would matter.