You can use neutral to autotransformer?

Yes, but the transformer must have both the hot and neutral to work as designed. A auto-transformer is not a variable resistor that could reduce current/voltage by a change in the resistance which could be used in the hot and or neutral wire. An auto-transformer is designed to have the hot wire and neutral on each end of the winding with a variable tap in between. The resistance of the transformer and the inductance will effect the circuit to a limited way if placed in the neutral but this effect will not vary the voltage over the full range, the amount of voltage reduction will vary with the load's current draw, see ohms law. Neither a resistor or a auto-transformer connected in the neutral line only will increase the voltage above the level of the input like a correctly connected auto-transformer . Stanley