I started a grails application by grails create-app. For modularity, I feel like it would be better that component be a plugin. Can I convert this application into a grails
I never created a plugin based on an application written before but looking at the documentation for grails plugins you can read the following statement:
The structure of a Grails plugin is exactly the same as a regular Grails project's directory structure, except that in the root of the plugin directory you will find a plugin Groovy file called the "plugin descriptor".
So I would suggest to create a new plugin with grails create-plugin *your-plugin-name*
and copy all files from your application into the plugin.