I would not add any casters. The casters on refrigirators are as soft as you can use and still move the frig. The problem comes when you over fill it. Do not try to move a full frig. The weight will dent the floor when rolled forward and back. You should never move the frig side to side. This is where most damage occures. Any wheel can gouge a floor when moved side ways.
"Modern" refrigerators have wheels that are almost useless in other words. Can someone (manufacturers) please put good wheels on 2 thousand dollar plus refrigerators. All they have to do is add more wheels that also turn thus spreading the weight and alleviating the necessity of scooting sideways. If the manufacturers are too stupid to do this it should be available in hardware stores as an add on. We should not be having to say that is the way it is. No wonder we are falling way behind in the world in almost every way. Trying to lift these behemoths to roll over plywood when they are up against the back wall is impossible