From memory (I am currently not with my Trainz computer) if you want to add additional commodities to a rolling stock item that the creator did not already assign it, then you will probably need to open the config.txt file of the rolling stock asset and add the commodity kuids to the product list. Not sure what parameters will be needed but check the other entries in the list for guidance. There could be problems with attachment points.
EDIT: The commodities that you add may not actually become visible on the railcar itself - a lot depends on the size, shape of the commodities, how old the asset is and how the asset creators set up the attachment points.