External Provider as Care Team Provider?

We are a specialty clinic that has a requirement to record and track a patient’s primary care provider separately from the referring provider. It occurs to me, perhaps naively, that despite the primary care provider being an external provider, that they are part of the patient’s care team.

We would like to set the external provider as the “Care Team (Provider)” for the patient, but the choices are limited to local users/providers. Does anyone know if there is a good reason why the choices are limited in this way? Is there a better way to track associated providers that aren’t referrers?

User setup has the concept of External Provider and is set in the Address Book.

Thank you @sjpadgett. Feels strange to add users with passwords (that will never use the system) for this purpose, but it should work. Do you know what ACL group is typically used in this case?