I had this idea a long time ago and found it hard to implement.
There is a lookup table for these platinum temperature sensors in the firmware, so at a first glance this will work. But a closer look will reveal the following situation:
ADC input of the Arduino is hardwired with V_Ref_low to Ground and V_Ref_High to VCC. That means, that the full range of 5V will be accepted as input voltage.
The resolution of the ADC is 12 bit, so the full range is divided by 2^12 , which ends up in a resolution 1,2 mV per LSB.
The thermal sensor is connected to ground (V_Ref_Low) and sourced by a 5KOhm resistor linked to V_CC (V_Ref_High).
Now have a look on the situation for a PT100: 100 Ohms @ 0°C. For this condition we will have a current through the resistor of 5V/5100 Ohms, approx. 1 mA. In this condition the voltage drop across the PT 100 is roughly 100 mV.
The correct calculation for the voltage across the PT 100 is:
V_PT100 = V_CC/(R_Bias + R_Temp) * R_Temp
with VCC = 5V, R_Bias = 5000 Ohms and R_Temp the resistor value.
Using the standard values from the PT100 gives for T=200°C a Resistance of 175,84 Ohms and a voltage drop of 169,866 mV, For 210 Degree the resistance is 179.51, the voltage drop is 172,288 V. In the region of interest you have a voltage variation of 3,4 mV = 2.8 LSB for 10°C, a resolution of 3.7 °C per LSB. This is enhanced by internal oversampling of 16 readings within the firmware, but this improves only by a factor of four. So at least you have a resolution of nearly 1°C for the temperature measurement, which is very poor for a good thermal control.
For this reason I switched to a PT1000 with 1KOhms bias resistor to improve the situation, but I found the temperature reading still too noisy. Therefore I came back to the original temperature sensor type but with a different housing and hotend design.
For the Thermocouples the voltage variations over temperature are much smaller, and I am not sure if they can be handled by the motherboard configuration correctly.
So my conclusion is: You can use PT100 (or PT1000) with the K8400 as a direct replacement, but without preamplifier the temperature sensing will degrade and the temperature control too. Thermocouples will not work without dedicated preamplifier.
The NTC is still the best temperature sensor, but the thermal link to the hotend must be improved. In either case, this must be the target for the replacement temperature sensor.