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
Yes. For Chromium use:
DefaultSelenium selenium = new DefaultSelenium("localhost", 4444, "*custom path/to/chromium" , "www.google.com");
selenium.start();
The other options that you can use are *custom, *chrome(note: this is not Google chrome, its a firefox mode only), *googlechrome, *iexplore. Please check selenium documentation for complete list of the modes.
EDIT: Changed googlechrome to chromium