Newbe here. I want to add information to an appointment or phone call that can be updated after it is closed like you can with notes. If I create a new entity, how do I link it to the appointment or phone call?

I want to add information to an appointment or phone call that can be updated after it is closed like you can with notes.  If I create a new entity, how do I link it to the appointment or phone call?  I would also like to have this new information on the left navigation panel.

I am using Microsoft Dynamics CRM 2011 online.