Let me answer my own question. Did some reasearch myself. At first there is no AD595 on my adapterboard. I don't know where that came from in my head. It is an INA826 Instrumention Amp which can also run on 3.3V. An AD595 or other 5V chip is a no-go on 3.3V 32bit controllers - the I/O ports can withstand 5V, but the A/D inputs are up to 3.3V.
Back to the INA826 adapterboard. I can hook it to one of the Thermistor Inputs and at the back of the S6 board there are foil jumpers with which the 4k7 pull up resistors can be disabled by cutting these. Then choose Thermistor #21 in Marlin and eventually assign new pins to the hotend thermistor in pins_fysetc_S6.h.
Option 2: use an Adafruit board with MAX31865 and hook it to the SPI/AUX3 connector.
Option 3: just use a Thermistor :-)