newer marlins comes with fake thermistor options that always just read what ever temp they are set to
* 998 : Dummy Table that ALWAYS reads 25°C or the temperature defined below.
* 999 : Dummy Table that ALWAYS reads 100°C or the temperature defined below.
// Dummy thermistor constant temperature readings, for use with 998 and 999
#define DUMMY_THERMISTOR_998_VALUE 25
#define DUMMY_THERMISTOR_999_VALUE 100
so all you need to do is set #define TEMP_SENSOR_0 999