by Tim Hunt.
The way you are supposed to it is to map {external_system}.id to mdl_user.idnumber. That is what the idnumber field is for, to have a place in Moodle to store externally significant IDs.
If you find the 'Show user identity' setting in the Moodle admin pages, then you can get the idnumber to show up in most reports (if that is something you need to do).