The general steps are, for USB mixer:
1. Connect microhpones, Skype output etc to a mixer.
2. Select the mixer as main playback device in RadioBOSS: http://manual.djsoft.net/radioboss/en/playback.htm
3. Select the input where mixer is connected in RadioBOSS for one of the line inputs: http://manual.djsoft.net/radioboss/en/microphone___line-in.htm
4. In broadcasting settings: http://manual.djsoft.net/radioboss/en/broadcast.htm set audio source to the line input where mixer is configured on step 3.