@Andreas, Thanks for posting in Q&A.
Currently, to enroll existing Azure AD joined devices to Intune, the options we can try are as below.
Option 1: Group Policy. (Local Group Policy)
Option 2. Registry
Option 3. PowerShell
Here is a link with detailed information you can refer.
https://call4cloud.nl/2020/05/intune-auto-mdm-enrollment-for-devices-already-azure-ad-joined/
Non-official, just for reference.
If there are a lot such devices, I think PowerShell script maybe more suitable for you.
Meanwhile, please ensure the Microsoft Intune licenses and Azure AD Premium license are assigned. And the MDM user scope under Automatic enrollment is set as all.
Hope it can help.
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