Hi there, MatthiasNi.

As my colleague shared above, you can sync them after you have enrolled to QuickBooks Online

I agree with the suggestion provided by Fiat Lux - ASIA, to use a third-party application. This is a tool that will help you synchronize your Datev and QuickBooks Online company.

To make sure the application is integrated to QuickBooks, you can check the tools available here at https://apps.intuit.com/app/apps/home.

For more information about migrating your data to QuickBooks Online, check out importing Data Conversion to QuickBooks Online - Frequently Asked Questions.

I'll be around if you need help. Keep safe!