Can you fit a Nobo NCU type thermostat cassette to a Nobo LST or C4N panel heater?

No.

The NCU type thermostats are physically different in size and electrical connections and cannot be fitted to the LST or C4N panel heaters. 

LST and C4N panel heaters used “Series 8” thermostat cassettes, all of which are now discontinued and are no longer available.