Do you mean like a swimable ring? If so, follow this, but for the first step, enter the name of your ring mesh.
http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=6878.0 (http://www.feral-heart.com/index.php?option=com_jfusion&Itemid=2&jfile=index.php&topic=6878.0)
Hope that helps. ;)