You can add search shortcut to your omnibox in Google chrome.
Right click on the omnibox and click Edit search engines.... You will get a dialogue box for editing search engines.

In other search engines section add Google search for superuser.com
- Give a name for your new search engine (Google Supeuser.com)
- Give a keyword for this (su)
- Give the url as http://www.google.com/search?q=site:superuser.com+%s

Here %s will be replaced by your search term. You can add any Google search operators to this.
Now you can use the keyword (su) for searching superuser.com.
Just type su in the omnibox and press tab key, and enter your search terms.
Note that on pressing the tab key Chrome will display the name given for the search engine (Google Supeuser.com).

This will show results from superuser.com alone.