It's possible to add or remove a plugin in the local_conf.py. Freevo will search the plugins the main plugin directory, the source directory and (based on the plugin name) in the media subdirectories like video or audio.
You should place your plugin into the main plugin directory if it doesn't depend on the media type. If it only works for e.g. video, you should place it in the video plugin directory. By doing that, the string video fill be part of the plugin name.
Everything inside Freevo is some sort of plugin. Without plugin, the main menu will be empty and Freevo couldn't play a file at all. So it's possible to change (mostly) everything in Freevo by writing a plugin.