It is now available with the latest PM2 version:

{ 
  apps : [{
    name : 'API',
    script : 'api.js',
    interpreter : 'node@6.9.1'
 }]
}

https://github.com/Unitech/pm2/blob/master/test/fixtures/nvm-node-version/ecosystem.json#L12

You can use environment variable.
For example:

1) NODE_PORT=3002 pm2 start -I 0 app.js

2) Read value in app:

console.log(process.env.NODE_PORT);

Or, if you are build express app:

1) PORT=3002 pm2 start -I 0 ./bin/www

2) Express load PORT automatically at start application.