You should really only submit an additional supplementary letter if there is a good reason. If, for example, you have volunteered regularly for the past couple of years at an organization, or you've had the same summer job for a couple/few years, you could ask your supervisor or boss to write you a letter. Bosses are able to speak to your reliability and hard work on a job site, and this conveys to the college that you are responsible and mature outside of the classroom. If you have an advisor who knows you personally–better than your classroom teachers–who can write a strong character letter (again - something new that the others wouldn't cover), then that is sometimes a possibility. Perhaps you've been studying an instrument or taking some other sort of class outside of school for several years, that instructor may write a letter about your commitment. The only reason you should submit an extra letter is if a person can speak to something about you that your teachers and counselors cannot.
There are two things, however, that you should avoid at all costs: a letter from a family member or close relation (no one will view it as an objective opinion of you), and inundating colleges with a slew of recommendation letters. That will hurt your chances more than it will help. Also, it's important to know that it won't hinder you in any any way not to have an additional letter.
Hope this helps!