Thank you, it worked out! I mean new ArduinoISP sketch was flashed correctly, next I would like to use it to program an ATtiny uC.
So I should replace every instance of Serial with SerialUSB and then use the programming port to upload the sketch ArduinoISP, after that connect the DUE board to the native port and flash the sketch on the external uC?
Do you think I can use the onboard SPI 6 pins?