@Michael Clarke, Thanks for posting in Q&A. From your description, I know you want to enroll existing Microsoft Entra joined devices into Intune. Based on my researching, we can use PowerShell script to do this. Here is a link with the detailed information for your reference:
https://call4cloud.nl/2020/05/intune-auto-mdm-enrollment-for-devices-already-azure-ad-joined/
Note: Non-Microsoft link, just for the reference.
Hope the above information can help.
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