I have another way for you to accomplish this in QuickBooks Online, Irincker.

At the moment, importing a customer list with account numbers isn't an option. You'll want to import the customer list first and manually enter the account number after. Alternatively, you can also use a third-party application that lets you import both sets of information. To search, type in a keyword on the search bar. Attached is a screenshot for your reference:

Furthermore, after you're able to successfully import customer information, learn how to run reports and customize them to get the data you need most. I've added this article for more information: Customize Reports in QuickBooks Online.

Let us know if you have any questions about importing the customer list and account numbers. We're always here to assist you further. Have a great day ahead.