I'm pretty sure what you submitted is final. You can call AADSAS and beg to have it included, it may be worth a shot. If it's a pretty important extracurricular (i.e. giving free dental work to indigenous Amazonian children while simultaneously providing them with health care and clean water), then you might send an email to your schools letting them know. If its not earth shattering, you might bring it up in the interview or just let it go. I mean, if you forgot it, how important is it?