Adding "thicker" steel is okay IF you can get the proper welds... Welding thick to thin or vice-versa is a problem because it is very difficult to get a consistant weld between two materials that are over 20% different in thickness. You will almost inherently end up with a too cold weld on the thick piece or to hot (read as blow through or holeing) on the thin. It is usually better to weld like thickness to like thickness. With weld consistency and quality a factor you will generally have the strongest assembly going this route.
Mark