I have one file that is my default file in `org-agenda-files` (which I set in my `init.el` file).

How can I add other files to the `org-agenda-files` list from within my default agenda file with paths relative to the default file?

Hypothetical example:

```
* One heading
  Some text
* Second heading (external)
  AGENDAFILE: ext/ext.org

```