3.5mm - mono connector to TRS jack type compatibility?
So, if I understand you correctly: you have a mono source with a 3.5mm TS jack and you want to connect this to an input with a stereo 3.5mm TRS jack.

You can simply build a cable with the L &amp; R channels tied together and connected to the mono signal. That is, sleeve to sleeve, tip on mono side connected to both tip and ring on the stereo side.

It will work fine; the audio will be mono.