When nRF52840 is powerd by USB, can I use the DECUSB pin as a 3.3V power supply?

[ndc_bee]

[haakonsh]

No, you cannot use the decoupling pin as a power supply. 

What you can do is connect the 5V USB supply to VDDH and configure it to supply 3.3V to VDD. See also Regulator specifications, REG0 stage. 

[ndc_bee]

I see the VDD output can be configured to 3.3V, now my question is can I use this 3.3V as a power supply for external sensor and MCU?

[haakonsh]

Yes, as long as you adhere to Regulator specifications, REG0 stage.