Reading voltage from USB pin on Argon, is it possible?

Not directly. You can connect the VUSB pin to an analog input, however you also need a resistor voltage divider or some other technique to reduce the 5V USB to 3.3V maximum for the ADC. There's a circuit in this recent post:

The other issue is that you will need to add a calibration factor. There's a Schottky diode between the USB 5V pin and VUSB that will produce a voltage drop of 0.1 to 0.2 V. It should remain relatively constant at room temperature so you can just calibrate it once.

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