It will void your warranty from many manufacturers, and some will even refuse to repair an out of warranty unit that was powered by a voltage converter. If you are talking about a new product [buying in Japan, and shipping to the USA], this is called a grey market product, as you are buying it outside of the manufacturers authorized distribution chain.
If you have moved from one country to another, and it is a used product, call the manufacturer, explain the situation, and ask if they can covert to the voltage standard where you now live.
If you have moved from one country to another, and it is a used product, call the manufacturer, explain the situation, and ask if they can covert to the voltage standard where you now live.