It is not possible to create a custom External ID for accounts configured in CloudHealth.
CloudHealth automatically generates a unique External ID for each account that is pulled in or configured in the platform. This is necessary for security purposes to prevent the "confused deputy" exploit, which is referenced here:
http://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_create_for-user_externalid.html#external-id-purpose