if we have reason to add more activities to our work/activities section, because we accidentally left them out, do most schools allow you to mail/e-mail supplemental information about the activities?
Most people don't list all their activities, even if there is extra space. Unless the experience is particularly relevant to your application (in which case how could you have forgotten it?), I wouldn't bother. Rather than e-mailing, I'd write about it in an update letter. Some schools specifically forbid supplemental communications (rare), so be sure that isn't the case before proceding.