If you're managing multiple domains from separate managed Google Accounts, such as G Suite or Cloud Identity, you might want to manage your domains from one account. That way, you can sign in to one Google Admin console to manage users and services for all your domains. And, users in all your domains can share Google services.

Before you begin

  • Important—When you delete an account from your Admin console, the deletion is permanent and the account cannot be restored. 
  • Consider performing this procedure when an extended amount of downtime won’t disrupt your business or organization. If you encounter issues, you might not be able to use your secondary or alias domain for a while.
  • Make sure you save all data in the accounts you’re deleting before you delete them. 
  • Choose the account you want to keep as the primary domain. Only the primary domain can have custom web addresses and domain aliases.
  • If a Google Play Developer account is associated with a user account name you want to move to a new domain, the user account name will not be associated with the Play Developer account once you delete it—even if you create a new user account with the same email address. You need to create a new Play Developer account using the new account name. Then, request to have the apps transferred to the new account. For more information, see Transfer apps to a different developer account.

Merge domains into one account

  1. Review the Before you begin information above. 
  2. Cancel your G Suite or Cloud Identity subscription and delete all but one of your accounts. For details, see Cancel your G Suite subscription or Cancel Cloud Identity and Remove a domain or domain alias.
  3. In the account you’re keeping, add the domains for all the accounts you deleted. For details, see Add more domains to your G Suite or Cloud Identity account. If the domain has its own set of users, add the domain as a separate domain. For details on domains and domain aliases, see Additional domains FAQ.
  4. If you added a domain as a separate domain because it has its own set of users, add the users to the account. When doing so, you'll be prompted to associate them with the domain they should belong to. For details, see Add users.
  5. If you saved the users' data from the old account, import their data back into their new account. For details, see Transfer data between G Suite accounts.

Source: https://support.google.com/a/answer/4417332?hl=en&ref_topic=6302293