Actually, there are some refits that make it possible for a slow BB to use CTNH. If an Emperor class battleship, say, had the Manoeuvring Thrusters refit (-5cm to minimum move distance before turning) and the Auxiliary Power Relays refit (+5cm speed) then it could, theoretically, use CTNH successfully since it now has 20cm maximum speed and only requires 10cm to turn. Unfortunately it specifically states in its rules that an Emperor class battleship can't use CTNH (no IN, SM, Chaos, Ork or 'Nid BB can).
It doesn't say that an Explorer can't though. So if an Explorer were to gain the two refits mentioned above then it could actually use CTNH.