I don't know if this is the best way, but my son emailed his Regional Coordinator (name/email address found in CIS) and copied to relavant school official and asked the school official to Reply All with confirmation. For example, he was elected captain of his team in the spring, and he emailed admissions and copied his coach. His coach replied affirmatively, and that was that.
While this is one method of doing this, I would recommend typing a letter with a description of what you are adding -- "from" the school official or the person who oversees the activity and "to" Admissions. Have that person sign the letter and then scan/email to Admissions requesting the new information be added to your record (also ask Admissions to confirm receipt).
This is a good way for a few reasons:
(1) If Admissions loses the email, you can simply resend the letter. You also don't have to worry about the coach or other person not responding or the email going to their spam folder. This letter can also be added to your own record (that you are hopefully keeping).
(2) You aren't flooding Admissions with a couple of emails, when it only takes one.
(3) Something in writing with a signature is normally more appreciated/formal (official) then just via email.
Example:
{Date}
Dear Naval Academy Admissions,
Joe T. Door (candidate number 123456) recently placed first in 1,600 meter run event at the Iowa state track and field championships with a school record time of 4:01. In addition, he also captured the regional and county championship titles. Enclosed are the results of these three championships.
Sincerely,
{signature}
Joe T. Coach
Head Track & Field Coach
Your High School
Enclosures:
(1) State Track Results
(2) Regional Track Results
(3) County Track Results