可以使用Apollo Federation的buildService函数来创建一个服务，然后将其提供给Apollo Gateway。在创建服务时，可以同时加载预先联合的schema。

示例代码：

const { ApolloGateway, RemoteGraphQLDataSource } = require('@apollo/gateway');
const { buildFederatedSchema } = require('@apollo/federation');
const { buildService } = require('@apollo/federation');

const service = buildService({
  name: 'products',
  url: 'http://localhost:4001/graphql',
  typeDefs,
  resolvers,
});

const gateway = new ApolloGateway({
  serviceList: [
    { name: 'accounts', url: 'http://localhost:4002/graphql' },
    { name: 'reviews', url: 'http://localhost:4003/graphql' },
    service,
  ],
  buildService({ url }) {
    return new RemoteGraphQLDataSource({
      url,
      willSendRequest({ request, context }) {
        request.http.headers.set('user-id', context.userID || '');
      },
    });
  },
});

(async () => {
  const { schema, executor } = await gateway.load();
  const federatedSchema = buildFederatedSchema([{ typeDefs, resolvers }]);
  // Use federatedSchema to get the pre-federated schema
})();