There is a file /home/user/mediaplayer-engine/radiochannels.m3u .
You can edit this file and add internet radio entries.
Download from a web radio station by right clicking a 'tune in' or similar link and it will download an .m3u / .pls file which contents you can add to the abovementioned file.
E.g. www.shoutcast.com has several 'tune in' links which start a media player by default but can download an .m3u (or .pls) file when right clicking.