Can I use the DECUSB pin as a 3.3V power supply to power up a sensor or something esle?
It's designed to power USB signalling interface.. why not to put an external $0.05 LDO?
PCB area is not enough, if DECUSB can be used as 3.3V power supply, it will be very helpful.
It can power USB signalling interface, theoretically it can power a sensor or a MCU?
or its current load capacity is not enough?
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.
Are you mean 5V USB supply to VDDH,then the VDD voltate can be configured to 3.3V, then I can use this 3.3V as a power supply for sensor or MCU?