Get external_id by user for associated accounts?

I see that there is an endpoint for fetching a user id by their external id (Discourse API Docs) but there doesn’t seem to be any way to perform the inverse operation. The user information object contains associated account information including provider name and email, but not the external ID associated with those accounts.

Is this functionality that could be added to the API or can the user object data be expanded to include the external id along with email for associated accounts?