Cell Phone Headset with 2 audio sources

[SJ360]

I have been toying with the thought of modifying a wired cell phone headset to accept (and mix) a signal from a mono audio source with its native output from the microphone. It seemed like something that would be easy to do with some splicing. After reading posts in other forums about mixing 2 sources to one output it doesn't seem so however and requires possibly resistors and a more profound knowledge of circuits etc.

I guess what I am asking; is this a simple mod...A mod that could be undertaken by a beginner? Or is this mod something that sounds simple but is in fact much more difficult?

Thanks for any advice offered as well.