You can't convert a SupaStox into a 12th car. As a quick look at both cars will show, there is nothing at all in common and the 12th parts will not fit.
Two best places to look for 12th car stuff is AMC (actionmodelcentre.co.uk) and Zen Racing (zen-racing.co.uk) and both will be happy to talk you through what you need. HTH
