I'm not even sure this can be done on Android, and if it is possible but complicated, it is probably not worth the time - but just in case...
Getting in and out of customized IEMs with over-the-ear bands is not exactly easy; it would be great if, when the following conditions are detected:
A headset with microphone is connected, either wired or wireless
Poweramp has audio focus
Playback is paused by either a button on the headset, or an on-screen Poweramp button
Poweramp could listen to the ambient sound through the headset microphone, and route it into the headset. This should stop when either:
Playback is resumed
Poweramp loses audio focus
Headset is disconnected
This would allow to have a conversation in headphones without removing them; kind of like what the blue button on Bose QC noise-canceling headphones does.