Is it possible to make a power auto-selector (ORing) with just a Diode and Mosfet?
No, it won't work that way.

First of all, you say N-channel, but the symbol in your schematic is a P-channel MOSFET. Look at how the body diode is pointed &mdash; it won't stop anything.

Even if it really is an N-channel device, the gate would have to be driven well above (Vth, to be specific) above the source in order to turn it on. No enhancement-mode FET will pass current (except through the body diode) with a "floating" gate.

Now, if you take your P-channel device and swap the drain and source terminals in your schematic, and add a resistor (100K or so) from the gate to ground, you'll have something that does something close to what you want. When the external power source is not present, the resistor will pull the gate to ground, relative to the source terminal at +3.3V, and the transistor will turn on. When the external power source is present, the gate will be driven to +5V, which will cut off the transistor, and the circuit will receive power through your external diode (but it won't be regulated to 3.3V).