Yes, you can import invoices to the sub-customers account, PGVicky.
Just make sure there's no space between the parent and the sub-customer when mapping the field. Here's a sample screenshot for your visual guide.
You can download this sample Excel file to help map the fields: Download the spreadsheet.
I'm also adding this article: Import multiple invoices at once. It provides instructions and detailed steps to guide you through the process.
Aside from importing invoices, you can also move suppliers, customers, stock, and chart of accounts. Please browse this link as your reference: Move your lists to QuickBooks Online.
Stay in touch with me if you have other concerns aside from import invoices. I'm always right here to help you.