@JC Zand ,
If I understand your question correctly, it sounds like you are trying to connect an existing Windows account to an Entra ID environment.
If this is what you are trying to do, here are the steps to do this:
On the Windows device, go to Settings > Accounts > Access work or school. Then select "Connect" enter your Entra ID credentials. Then you should see your Entra ID account listed under "Access work or school." Under your Entra ID account > Info > Manage > Connect to work or school, you can select "Join this device to Microsoft Entra ID".
Once the device is enrolled, you should be able to access resources with the Windows account.
https://learn.microsoft.com/en-us/entra/identity/devices/device-join-out-of-box
Do let me know though if there is more you would like to accomplish and if I understood your full question.
If the information helped you, please Accept the answer. This will help us as well as others in the community who may be researching similar questions.