Hello @koreanEggCoffee

yes, that is possible - I am powering my M5Atom this way.

However please note: There is no protection against wrong polarity so please be careful not to accidentally swapping 5 volts and GND.

I also suggest you disconnect the external 5 volts before connecting USB-C. Or in other words only connect one power source at the same time.

Please also refer to the M5Atom schematics which outlines the 5 volts connections between USB, 5v pin, Groove, LEDs and 3.3 volts regulator.