Hello, my name is***** to Justanswer! Give me just a few minutes to look up some specifics, I'm working on this now.
The easiest way is to use a computer program. Your recorder isn't built as a mixing station, it's a recorder. There are several free mixing programs out there, click here for Wavepad. That will allow you to multitrack the wav and mp3 files, and will mix it down into one audio track.