How can I add a new extension type so it'd be listed in the Navigators and open dialogs?

Zoom Player keeps a list of valid file Extensions (".mpg", ".avi", etc...) so it would know which files are media files and should be listed and added to the play list (when opening entire drives or directories). You can add new extensions to this list at any time by following this procedure:

Open the Options Dialog, go over to the "System / File Extensions" page. You should see the file extension categories: one for Video files, one for Audio files, one for Subtitle files, one for Play Lists and one for Executables. Simply type in your desired extension at the end of the line and make sure each entry is separated by a ";" character.

Now all dialogs and commands that filter by extension should display all files with the extension you specified.

The Execution and Subtitle extensions are handled a bit differently, they won't open directly, but are rather used for highlighting and detection of files.