You can make this work in one of three ways:
1-You can make just a regular plugin which detects the active source and pauses it before starting to play audio and you can control the radio with your own commands and screen/skin.
2-You can make a custom source and control the radio with your own commands and screen/skin. I believe mitch made the source code for one of his source plugins available as an example.
3-You can make an actual RADIO plugin and any existing radio skins will work with it directly. Check your PM - I can send you a sample radio plugin source.