The problem here is that "UN" is always definite, therefore you cannot at a possessed entity before it without also making it definite.How do we differentiate between a definitive noun and an non-definitive noun in اضافة؟
For example how would I differentiate "a delegate of the UN ..." (مندوب الأمم المتحدة) with "the delegate of the UN" ( a specific delegate)?
I am curious about this as well. I am always uncertain about when to use ل to indicate possession rather than the typical possessive ending.The problem here is that "UN" is always definite, therefore you cannot at a possessed entity before it without also making it definite.
In general, you can solve this problem with the preposition ل, although Cherine's suggestions, and others like them, are sometimes more common.
For a simple example, 'my friend' is صديقي. Since 'my' is always definite, to say 'a friend of mine' you have to use the prepositional phrase to create a synthetic construct:
So what you want literally, whether it's used or not, is مندوب للأمم المتّحدة
I'll leave it to native speakers to tell whether this is مستساغ or not.