Presumably you mean this volume control indicator (as there's two versions):
This would have to be a feature request upstream to the maintainers. Bear in mind by design, it's intended to be a simple menu for adjusting the main volume and start/stopping playback. A change upstream would also impact other flavours/Ubuntu distros, too.
That said, nothing is stopping someone from forking the code and making the modifications themselves.
MATE Volume Control has the options for per-app volume control. You can get to this screen in 3 clicks by clicking the indicator, clicking Sound Preferences and then the Applications tab:
If you create a launcher on the panel, you could actually get to this screen in just one click using this command: