You can try to use “Move” method that available in menuObject.Items.
The snippet below shows how to add a row context menu item and a separator to the top position of WebGrid row context menu.

function WebGrid1_OnRowContextMenu(controlId, rowType, rowElement, menuObject) {
    var separator = new WebMenuItem();
    separator.Type = "Separator";
    separator.Name = "MySeparator";

    var menuItem = new WebMenuItem();
    menuItem.Text = "View Row's KeyValue";
    menuItem.Name = "MyMenu";
    menuItem.OnClick = "RowKeyValue";

    //Add Menu Separator and Menu Item
    menuObject.Items.Add(menuItem);
    menuObject.Items.Add(separator);
    menuObject.Items.Move(menuItem.Index, 0);
    menuObject.Items.Move(separator.Index, 1);
            
    return true;
}

Hope this helps.