Hi @Paul S
Thanks for reaching out to Microsoft Q&A.
If your device is registered, and you have an Entra ID account, the device will use an artifact called "PRT" (Primary Refresh Token) for SSO. Since the device is already registered, there's no need to sync it from your onpremises infra. You can definitely enable SSSO on your Entra ID Connect though as it won't do any harm to your current environment.
Let me know if you have any questions.
Thanks,
Fabio