So you need to fill your users' ID Number with the same value you'll have in your SIS. For example, with fill our users' ID Number with LDAP's 'employeeID' attribute.
We have stored there their IDAL (IDentificador de ALumno -- student identifier in Spanish) in the 'employeeID' attribute and then we pull this same value (IDAL) from our SIS database in the externa database enrolmente plugin. This way Moodle can correlate Moodle users and SIS users.
I hope this helps.
Saludos. Iñaki.