Intune registration for existing hybrid Join Windows device

Your best bet is to enroll it in the "https://businessstore.microsoft.com/en-us/manage/devices/all" and add it under the devices tab. You will need a .csv file with the device hardware hash which you can acquire from these instructions https://learn.microsoft.com/en-us/autopilot/add-devices.  Then go to "https://intune.microsoft.com/#home", devices, enroll devices, and under "Windows Autopilot Deployment Program" click on "Devices". You may have to click "Sync" and give it some time but from there you will be exclusively in Intune. Hope this helps! Let me know if you have any trouble.