My advice would be to shoot the WikiTree team an email describing the issue and asking them to either manually change the surname to "الخوري" or to delete the "Khoury" profile so that you can create/use a new "الخوري" profile. I can't really think of any other solutions that you could use on your side. I just changed the surnames of a few of my Lebanese ancestors to their original arabic spelling, without any problems, so it is probably (as you have already mentioned) just a problem with this specific profile.