Generally speaking, your LSDAS GPA is the number that law schools will be looking at when considering your numbers. But, like any other aspect of your application, if you have a weak/troublesome area that you feel should be brought to the admissions committee's attention, you may wish to submit an addendum.
A well drafted addendum may help an admissions committee understand a certain aspect of your application; however, like the poster above mentioned, you will need to have some sort of mitigating circumstances for the committee to hang their hat on. No matter how well you draft your addendum, if you do not have reason why you received your "NPs," it is not likely that the committee will disregard the LSDAS GPA in favor of yours. If you do indeed "have fairly good explanations for why [you] didn't pass [those] classes," then it may be worth it to write a letter.
But again, an addendum typically helps an admissions committee understand a certain part of your application. Even a valid reason for the shortcoming is no guarantee that the committee will disregard the problem area.