If you are trying to build a url, you can always slightly bypass the MVC routing and just pass the complete url in the old-fashioned way.
string dataParam="hello";
int otherParam=5;
return Redirect( String.Format("/Index?data-param={0}&data-param2={1}", dataParam, otherParam) );
If you are going outside of your own MVC application then you may not want RedirectToAction
anyway unless you are redirecting to an action.