Not when the engine is hot or you risk unevenly quenching the head and block and warping or cracking those. OK if cool/cold. If the radiator is empty and you just fill the overflow tank, the radiator will still be empty. Coolant gets sucked into the radiator from the overflow as the engine cools and expanded hot coolant shrinks in volume forming a vacuum.
Always put coolant in the plastic reservoir, assuming your car has one. If your car is old enough not to have a reservoir, the engine should off and the radiator cap should be cool to the touch.
Off and cool if you wish to put it in the radiator, anytime if you put it in the reservoir.
