Change field type

Change field type

In the "Calls" module (which I believe is a subset of Activities) I want to either change the field Call Result from Text (255 character limit) to Text Area (64,000 character limit) OR add a custom field that I will use to replace Call Result.  Unfortunately, I can not find a way to do either.  I can add custom fields to the any other modules, but not Calls.  This does not make a lot of sense, so can someone get back to me with a solution?