No. You can only create a link from a user who has never signed in. That is, you can only link the new user at the point they are created (in the pre-auth trigger).

"This allows you to create a link from the existing user account to an external federated user identity that has not yet been used to sign in".

Personally I catch this case in the pre-signup trigger, and reject the sign up with a custom message ("An account already exists with this email address, please sign in using Google").

Here is my pre-signup lambda in case you find it useful

const AWS = require("aws-sdk");
const cognito = new AWS.CognitoIdentityServiceProvider();

exports.handler = (event, context, callback) => {

  function checkForExistingUsers(event, linkToExistingUser) {

    console.log("Executing checkForExistingUsers");

    var params = {
      UserPoolId: event.userPoolId,
      AttributesToGet: ['sub', 'email'],
      Filter: "email = \"" + event.request.userAttributes.email + "\""
    };

    return new Promise((resolve, reject) =>
      cognito.listUsers(params, (err, result) => {
        if (err) {
          reject(err);
          return;
        }
        if (result && result.Users && result.Users[0] && result.Users[0].Username && linkToExistingUser) {
          console.log("Found existing users: ", result.Users);
          if (result.Users.length > 1){
            result.Users.sort((a, b) => (a.UserCreateDate > b.UserCreateDate) ? 1 : -1);
            console.log("Found more than one existing users. Ordered by createdDate: ", result.Users);
          }
          linkUser(result.Users[0].Username, event).then(result => {
              resolve(result);
            })
            .catch(error => {
              reject(err);
              return;
            });
        } else {
          resolve(result);
        }

      })
    );

  }

  function linkUser(sub, event) {
    console.log("Linking user accounts with target sub: " + sub + "and event: ", event);

    //By default, assume the existing account is a Cognito username/password
    var destinationProvider = "Cognito";
    var destinationSub = sub;
    //If the existing user is in fact an external user (Xero etc), override the the provider
    if (sub.includes("_")) {
      destinationProvider = sub.split("_")[0];
      destinationSub = sub.split("_")[1];
    }
    var params = {
      DestinationUser: {
        ProviderAttributeValue: destinationSub,
        ProviderName: destinationProvider
      },
      SourceUser: {
        ProviderAttributeName: 'Cognito_Subject',
        ProviderAttributeValue: event.userName.split("_")[1],
        ProviderName: event.userName.split("_")[0]
      },
      UserPoolId: event.userPoolId
    };
    console.log("Parameters for adminLinkProviderForUser: ", params);
    return new Promise((resolve, reject) =>
      cognito.adminLinkProviderForUser(params, (err, result) => {
        if (err) {
          console.log("Error encountered whilst linking users: ", err);
          reject(err);
          return;
        }
        console.log("Successfully linked users.");
        resolve(result);
      })
    );
  }

  console.log(JSON.stringify(event));

  if (event.triggerSource == "PreSignUp_SignUp" || event.triggerSource == "PreSignUp_AdminCreateUser") {

    checkForExistingUsers(event, false).then(result => {
        if (result != null && result.Users != null && result.Users[0] != null) {
          console.log("Found at least one existing account with that email address: ", result);
          console.log("Rejecting sign-up");
          //prevent sign-up
          callback("An external provider account alreadys exists for that email address", null);
        } else {
          //proceed with sign-up
          callback(null, event);
        }
      })
      .catch(error => {
        console.log("Error checking for existing users: ", error);
        //proceed with sign-up
        callback(null, event);
      });

  }

  if (event.triggerSource == "PreSignUp_ExternalProvider") {

    checkForExistingUsers(event, true).then(result => {
        console.log("Completed looking up users and linking them: ", result);
        callback(null, event);
      })
      .catch(error => {
        console.log("Error checking for existing users: ", error);
        //proceed with sign-up
        callback(null, event);
      });

  }

};