1. 创建一个Lambda函数并设置为AWS Cognito Pre-SignUp Trigger。
2. 在Lambda函数中，通过使用AWS SDK调用AWS Cognito的AdminLinkProviderForUser函数，将已有的社交媒体帐户链接到用户的Cognito身份池帐户中。
3. 下面是示例代码：

exports.handler = async (event) => {
    const AWS = require('aws-sdk');
    const cognito = new AWS.CognitoIdentityServiceProvider({apiVersion: '2016-04-18'});

    const {userName, request, response} = event;

    if (request.userAttributes.hasOwnProperty('email')) {
         const email = request.userAttributes.email;

         const providerName = 'graph.facebook.com';
         const providerAttributeName = 'Facebook';
         const providerAttributeValue = request.userAttributes['custom:facebook_id'];

         const providerDetails = {[providerAttributeName]: providerAttributeValue};
         const params = {
             DestinationUser: {
                 ProviderAttributeName: 'email',
                 ProviderAttributeValue: email,
                 ProviderName: 'Cognito'
             },
             SourceUser: {
                 ProviderAttributeName: providerAttributeName,
                 ProviderAttributeValue: providerAttributeValue,
                 ProviderName: providerName
             },
             UserPoolId: event.userPoolId
         };

         const result = await cognito.adminLinkProviderForUser(params).promise();

         console.log('Linked account', userName, result);
    }

    return event;
};