The invoicing feature in QuickBooks Self-Employed is a simplified version. As of this time, importing your list of invoices into QuickBooks is not available. Also, you can only import bank transactions and tracked mileage.
On the other hand, I can see in how beneficial it is for you and your business to have the ability to import your invoices. I'd personally share this to our Product Engineers for future product update considerations. To keep up with what's new in QuickBooks, you can visit our blog: https://intuit.me/2L2zqTv.
Know that you're always welcome to post if you have any other concerns. Wishing you and your business continued success.