To add a single environment variable, You can do something like this.
[program:django]
environment=SITE=domain1
command = python manage.py command
But, if you want to export multiple environment variables, you need to separate them by comma.
[program:django]
environment =
SITE=domain1,
DJANGO_SETTINGS_MODULE=foo.settings.local,
DB_USER=foo,
DB_PASS=bar
command = python manage.py command