I'm trying to add Custom Fields for the Contact area. I know I can create custom data for a set of custom fields I create myself, but I can't seem to add custom fields to pre-existing sets created by CiviCRM itself. How can I handle this? Or must I only add to Custom Sets I create?
You create custom fields in custom field sets. You have the choice of them being displayed inline or tabs, and as initially collapsed or open, but beyond that, you have little control over where they are displayed on the contact summary.
However, with a bit of jQuery they can be moved around. For an example (although not custom fields) see https://civicrm.org/extensions/group2summary