A coalition that includes doctors, environmentalists, Eskimos and anti-nuclear activists said it will fight a U.S.-Japan agreement allowing international shipment of plutonium through Alaska. The state of Alaska already is fighting the Reagan Administration in federal court over the plutonium flights. The coalition said its efforts would be aimed at getting a groundswell of public opposition in Alaska and Canada and in Congress, where the agreement can be ratified, amended or rejected. The U.S.-Japan Agreement for Nuclear Cooperation, which was signed in Tokyo last week but still must be approved by the Japanese Diet as well as Congress, and the U.S.-EURATOM (European Atomic Energy Community) Subsequent Arrangement would allow Japan to have spent nuclear power plant fuel reprocessed in Europe for shipment back to Japan via Alaska. The flights also might go over Canada.