Would it be appropriate to submit a GPA addendum written by a faculty advisor? Basically the advisor is aware of the health issues I experiences during my undergraduate and is willing to prepare a short 'to whom it may concern' letter that outlines how my health setback has affected my grades.

Is this appropriate? Or would law schools prefer that I wrote the addendum myself? Would this perhaps accompany a statement written myself?

Thanks in advance!

No. You should write a GPA addendum yourself. The only part of your app that should be written by someone else is the LORs (or a Dean's Certification, if required).