See Diagram:
If the destination server (meaning the device that you are NATing for) is immediately behind the FW then yes, the "Next Hop" would be the server itself (see Diagram - situation (a)).
I can remember the exact SPLAT syntax atm - but it would be something like...
route add -host [NATADDRESS/32] gw [DEST SERVER ADDRESS] -
Alternatively Add Route via the SPLAT Web Interface If the destination server is behind an internal routing device then the route would point towards the internal router, as the next hop, instead (see Diagram - situation (b)).
route add -host [NATADDRESS/32] gw [INTERNAL ROUTER ADDRESS] -
Alternatively Add Route via the SPLAT Web Interface