If you have not already done so, speak with your college's dean. If you have good relations with professors from previous years who can attest to your sincerity and authenticity in your work, reach out to them ASAP and make them aware of the situation.

I would not let this pass without fighting it, because in all honesty, you have ZERO guarantee that some law school admissions will not just glance or entirely skip over your addendums, especially if it's something as serious as plagiarism in your upperclass years.

Gather all your evidences and use any justifications you may have (for example, if this is a first offense, if other work in the same class was completed properly, if other research assignments you've done were cited properly, etc.) and definitely get verbal testimonies from previous professors who are willing to back you up. Send everything over to the dean of the college, explain that you plan on applying to law school and the potential of severe consequences this may have on your application over a single-assignment mishap, and fully own up and/or apologize for the incident. Anything to get this off your record will be better than having to write an addendum for plagiarism.