I want to know how to tag an existing customer to a new one where the new customer which I have not registered yet would be the parent customer.

Glad to hear from you today, abayomi.

I can share some information about adding a parent customer in QuickBooks Online (QBO).

You can add the new parent customer and simply edit your previous top-level customer to become a sub-customer below this.  

Here's how:
 

1. Go to Get paid & pay and select Customers (Take me there).
2. Choose New customer.
3. In the Display name as field, enter what you want to display for the customer. (This is a required field.)
4. Then, review each section and enter any other important customer info.
5. Select Save.

After creating the parent customer, you'll have the option to create a sub-customer, you can check this article for more details: Add and manage customers in QuickBooks Online.

On the other hand, you can record transactions in two ways. You can either create a sales receipt or an invoice. To know more about this, I've added this article for more details: Record Invoice Payments In QuickBooks Online.

Reach out to us if you have any concerns about your customers. Remember, we're here to make sure everything is taken care of.