Are you talking about those pre-made bath tub slip cover thingies? If so, yes, they will just pop over the entire old bathtub. Just make sure that you get the correct measurements and if there are any windows over the unit, that they are cut to fit perfectly.
http://t2.gstatic.com/images?q=tbn:ANd9GcTdkBGPr2zbnNLjj1tzBSj15P3v7TohZoPpj-oaSSFblxk0jvg&t=1&usg=__MVas_k_7V_yJDHKa67cWpk9xQbQ=