There are several options that will have this result: first, you have the option of creating a new background label, to which you can assign a degree ranking boost or other behavior. See how to add new background labels in our documentation: http://www.google.com/coop/docs/cse/cse_file.html. Note: Choosing to ignore a previous background label, as described in the Getting Started Guide linked above, will only work for the free standard edition of Custom Search and is not supported in the business edition. Users will get a 403 error when they apply this particular refinement to your Google Site Search.
Your second option is to append the
more: operator (followed by the label name) automatically to your users' queries in order to trigger one of your labels. You can do this by altering the search form from which users submit their queries. You might want to keep this query visible to the user so they can remove it if necessary.
Finally, some of Google advanced search operators might allow for the behavior you're looking for. To learn more about advanced search operators, you can visit http://www.google.com/help/operators.html.