Export all stations
There's an "Export All" plugin available. It adds an entry under Main menu ▸ Extensions ▸ Export all stations. Which allows to export all stations from the currently visible channel category.
This is of course intended primarily for saving your bookmarks. And it allows to import radio lists into any other music player or playlist collection/management app (such as Rhythmbox).
Configuration option
You can set the default export format in the settings dialog F12.
It supports all common export formats now, with .PLS being the default.