Referencing existing env vars is done with %(ENV_VARNAME)s
See: https://github.com/Supervisor/supervisor/blob/master/supervisor/skel/sample.conf
Setting multiple environment variables is done by separating them with commas
See: http://supervisord.org/subprocess.html#subprocess-environment
Try:
environment=PYTHONPATH=/opt/mypypath:%(ENV_PYTHONPATH)s,PATH=/opt/mypath:%(ENV_PATH)s