I tend to assume it is disorder because it can be "treated" or "cured" with modern medicine, if it is in fact not some sort of disorder I do not think Genetic modification could alter this since there would not be a gene.
I am not in favor of changing personalities with Genetic Modification, nor do i believe such a thing is practical. I think that in General in society however someone with something like ADD could probably live a less stressed life without it and that if it is a disorder it is no immoral top remove it from the time of birth. In a case like this I suppose that hypothetically it should be left individually to the parents.