Hi @tg-taxoa-com 

I know it is not supported in QuickBooks Online to specify the Parent Customer when importing into the Customer list but you can do that if you use one of the data import utilities that you can find in the Intuit App Marketplace like Zed Axis (desktop edition).  

In particular when it comes to Customers with multiple levels you can import up to five levels deep for example

Parent:Customer:Job:Sub Job:Sub Job

There is a 30 day free trial you can download and test this out if you need.  Hope that gives you solution.