Rest WS Java: core_user_create_users()

Hi guys, I'm trying to integrate my Java App with Moodle 2.8.

I was testing some typical issues like create categories, courses and users when I found some problems. For example, the API documentation for core_user_create_users() shows we can set an 'idnumber' for a user. I did correctly, no mistakes, but when I get this user with core_user_get_users(), this field appears as null.

I've tried looking for the 'idnumber' and for the automatic 'id' Moodle provides, but it keeps failing. I need for mapping users' id from my app's database with Moodle's.

Thanks ;)