I am using the Biblio module and I like the display mode (e.g. full, teaser). I would like to tack on custom text and information when a user views the content below the display. How do I append my own custom text in the current display? When I go to "Manage Display", the custom fields are not shown. It might be hard coded in the module. What Drupal API call should I be looking for? Or better yet, I am perfectly willing to write my own module. I just do not know where to start in adding my own additions to an existing display.

An example is the comment module. When someone leaves a comment, then you can see the comment someone has written that is tied to the specific node and/or content type. I am not talking about creating a user input form or input HTML field.