Adding bluetooth speakers as rear channels - Pulseaudio configuration

Hello, I am basically trying to recreate setup from This thread

Currently I am using stereo speakers in 2.0 configuration via HDMI and I am trying to utilize another set of BT speakers as surrounds. I tried to follow directions from abovementioned thread but when I get to the part where they´re editing default.pa in etc/pulse… I realized that these files are read only on Coreelec. Is there any other way to achieve this?
Thanks in advance!