In the Selenium options (on Firefox) I can find Custom browser.
Is it possible to use this option to run a Selenium test in Chromiu
Uh, the accepted answer doesn't answer the question. Google Chrome is based on Chromium, but they're not the same browser.
This is what you want: (since Chromium isn't officially supported)
DefaultSelenium selenium = new DefaultSelenium("localhost", 4444, "*custom C:/path/to/chromium.exe" , "www.google.com");
selenium.start();
Edit 2018-08: Looks like the accepted answer changed to a copy of this one several years later, so my original comment is no longer correct. I'm leaving it there, but struck out, because the votes are misleading if I straight remove it.