Adding a LDAP link in work item Defect in RTC

Based on your statement it sounds like the user reporting the defect isn't doing so by creating a work item, so their name is not attached to it as the creator. So  you want to create a field where the value is linked to an specific user in LDAP but not necessarily one that has been added to any RTC groups. This way the user creating the defect is forced to use a standard naming convention.
Is that correct?

I don't know the answer but if we can clarify the question a little more then maybe someone else will.

Karl, 

 I think Piotr you might just have the solution for me. I already created a custom field contributor and I see I get the list I want. Now I am trying to find a way to do a bulk import to get all users, but I don't see an answer at the moment. Google doesn't help a lot. So I'll keep searching, but if somebody knows how or knows the right link please let me know. 

I already tried this way, but it seems to be limited to 1 person per import. Can't find an explanation of bulk import and I didn't figure it out yet. Still searching.