Reflector 3 can add voice-over audio to device recordings. The microphone you use can be any microphone connected to your computer. Your device microphone cannot be used.
Changing the microphone input is easy and can be done before beginning a recording. Select the microphone icon in your Reflector menu, then choose the microphone that you will use for the recording.
Clicking the down arrow next to the microphone will provide a list of microphone choices.