Thanks a lot.
I probably won't succeed in adding a working Navmesh to those (as stated in the OP, every attemps in the past resulted in CTDs and waste of time without me having a clue what went wrong). But I might give it a shot.
Although, the reason why nobody did this before is probably that they aren't allowed to. If I recall correctly from my time within the Forgotten Honor campaigns, those maps are strictly not to be distributed further in any form.