Every user now carries a metadata field: a free-form map of your own keys and values. Hang an external reference straight off the Examplary user — the id for a person in your SIS, LMS or CRM — instead of keeping a separate mapping table on your side.
Set it when you create a user or with PATCH /users/{id} (needs users:write), and read it back from GET /users or a single user (users:read). A patch merges into whatever is already there, and setting a key to null clears it. Values are scalars — a string, number or boolean — not nested JSON.