There's a plugin that I would like to add features since it is not quite usable (in a practical way). Do you create a new plugin or is it allowed to update the existing one?
You should be able to add to an existing one. Ideally the plugin's page on the plugin portal ( http://grails.org/plugins/) should have a link to the source which is usually in subversion or git. If it's in Subversion or a private GIT repo you might have to email the grails developer list ( http://grails.org/Mailing+lists) and ask for permission to the repo. If it's in a public GIT repo you should be able to fork it, make your changes, and submit a pull request.
For example the Grails Cloud Foundry plugin ( http://grails.org/plugin/cloud-foundry) source code is located at ( https://github.com/grails-plugins/grails-cloud-foundry)
If for some reason whoever owns the repo or plugin isn't working on it anymore it may make sense to create your own. ( http://grails.org/Creating+Plugins)