Yes, this is possible!
To add custom keys, just put them in there. Errai's configurable parts ignore properties they don't understand. I would recommend you avoid making up properties names that start with "errai." and instead choose a prefix (such as "org.mycompany.") to avoid accidentally colliding with future Errai features, or future 3rd party Errai extensions.
To retrieve the values within your app, you can call org.jboss.errai.config.rebind.EnvUtil.getErraiAppProperties() at rebind time, iterate over the returned collection of Properties instances (one for each ErraiApp.properties on the classpath), and pull out the configuration data you need.