There is no way directly from QB's that I know of. But you can open your customer center, and at the top click the Excel tab, and export the customer list to excel. Then in excel, you can sort the email column alphabetically, and then select all the email addresses, hit Ctrl C to copy them, then open your started email, and past these addresses in the BCC field.
This is not something that we want you to experience, @MargauxInDenver.
That being said, did you encounter any error with converting your QuickBooks Desktop company into QuickBooks Online? This is for me to provide you with an accurate resolution.
In the meantime, you can read through these articles for the step-by-step process: Move your QuickBooks Desktop file to QuickBooks Online.
You can always visit the Community site if you have other concerns, or if there’s anything else you need. I'm always here to help. Wishing you the best.