Fake thermostat with resistor
First of all, PT100 is not thermostat, its a type of resistance thermometer. And that by the way looks like an PTC thermistor.
Technically yes you can do so, It's very wrong and dangerous but you can do that. You are asking why it's dangerous? well, your heating device is using the resistance thermometer to get feedback from real temperature. When you are replacing the temperature sensor with a fixed resistor, you are breaking the feedback loop. the device is designed to read the from that certain type of sensor, let's assume the resistance of the sensor is 100K for say 20 &deg;C, and you are replacing the sensor with a fixed 100K resistor, and set the goal temperature to 20 &deg;C, the device is probably going to deceived into turning the heating system off, but problem starts when your resistor's value for any reason starts change (because they are not ideal), heating system will try to increase or decrease the temperature to meet the desired temperature, which will never be reached, and that will result in system damage.  
if you want to replace a resistance sensor with a resistor make sure that the resistor is completely fixed, otherwise you are going to end up with a highly unstable open-loop system with a large gain on the input.