Obviously .ecm isn't listed as a viable extension for PSX, but if you're sure that it will open, give it a go and be sure to report back with your results. If it works, this might need to be added by default for all users to take advantage of.
To begin, you shouldn't edit that file directly. Make a copy to the alternative directory with:
cp /etc/emulationstation/es_systems.cfg /opt/retropie/configs/all/emulationstation/Then you can edit it with:
nano /opt/retropie/configs/all/emulationstation/es_systems.cfgFrom there, the systems are listed alphabetically, so work your way down to 'psx' and change:
<extension>.cue .cbn .img .iso .m3u .mdf .pbp .toc .z .znx .CUE .CBN .IMG .ISO .M3U .MDF .PBP .TOC .Z .ZNX</extension>to:
<extension>.cue .cbn .ecm .img .iso .m3u .mdf .pbp .toc .z .znx .CUE .CBN .ECM .IMG .ISO .M3U .MDF .PBP .TOC .Z .ZNX</extension>After you've made your edits, press 'ctrl+o' to save and 'enter' to confirm.