Hi Djulia,
Okay, I see. There aren't a lot of simple options. One approach would be to have an admin drop down field that only showed authorized users using an advanced MySQL query. This field could be set by a custom plugin when a new record is selected. This field could now be used in the advanced search. Outside of that, you would need to actually customize the core CMSB code in order to make a special case for this search field.
Hope this helps
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