N/A.
Contacts do not have a system-generated unique UserID, such as a SID. This unique ID is required for CA to add the Contact as a synchronized Employee and for DA to add the Contact as a synchronized Custodian.
This is expected behavior. To add a Contact to CA and DA, the contact's information must be used to manually create the CA Employee or DA Custodian. Such manual creation needs to ensure the check box for synchronizing is unchecked as the account cannot be synchronized. Any changes to the Contact's information, such as email addresses, must also be manually updated in CA and DA.