@andreas bright , Agree with Jason, to convert existing windows 10 devices which are not registered as an Autopilot device, we can create an Autopilot deployment profile, set "Convert all targeted devices to AutoPilot" to Yes. Assign this profile to the device group which include the devices we want to convert. After some time, check the "Assigned devices" under the profile to see if they are visible there. If yes, it means these devices are Autopilot ready. We can reset the device to do Autopilot. Here is an article with detailed steps for the reference:
https://www.robinhobo.com/automatic-add-existing-windows-10-devices-to-windows-autopilot/
Note: Non-Microsoft link, just for the reference.
Hope it can help.
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