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!
