How can I add an item to a SelectList in ASP.net MVC

There really isn't a need to do this unless you insist on the value of 0. The HtmlHelper DropDownList extension allows you to set an option label that shows up as the initial value in the select with a null value. Simply use one of the DropDownList signatures that has the option label.

<%= Html.DropDownList( "DropDownValue",
                       (IEnumerable<SelectListItem>)ViewData["Menu"],
                        "-- Select One --" ) %>

I got this to work by Populating a SelectListItem, converting to an List, and adding a value at index 0.

List<SelectListItem> items = new SelectList(CurrentViewSetups, "SetupId", "SetupName", setupid).ToList(); 
items.Insert(0, (new SelectListItem { Text = "[None]", Value = "0" }));
ViewData["SetupsSelectList"] = items;