How to make mix mono into stereo audio
No, using diodes does not accomplish what you want.  That will partically rectify the additional signal, making it sound like a mess.

You need to actually mix the new signal onto each of the two stereo signals separately.  That might be as simple as a few resistors if the new signal is well buffered so that its impedance is low, and whatever is receiving the mixed signals can handle high input impedance, like a few 100 Ohms.