How to programmatically design mode select a component to be included in the sidekick

The problem you want to have a component programmatically available to authors without having to enter design mode and add a component for the template.

The solution Go to your relevant template's design mode. For this example, we 

will use the Geometrixx site, located at:
/etc/designs/geometrixx/jcr:content/contentpage/par

Then edit the components property to contain the path to your component, for 

Example:
/apps/geometrixx/components/customcomponent

Save, and reload the editor, and you should have the component added to your list.