Adding a Custom Field

I would like to add a custom field to the Advanced Fields section in the Add Fields menu when creating/editing a form. Is there any way to do this or are those fixed by the plugin? I am looking to do this without editing any of the code that comes with the Gravity Forms plugin but rather write another plugin that will cause the newly added fields to show up. My goal is to make this plugin available to some of my clients that would like to integrate our CAPTCHA service onto their Gravity Forms with ease. Thanks in advance!