All the information regarding customizing vehicles can be found in the PaintKit documentation. However, you can only use the models that are already provided and/or configure their liveries, not add new models.
However, there are already different styles of vehicles ( EU and USA style ) and they are already assigned to Europe and USA so, I'm not sure exactly what case you are referring to. Please make an example of an airport (clearly indicating the gate AND the airplane used too ), in Europe were you got an american catering truck or vice-versa, so I can explain why it happened, most likely another constraint like parking size, airplane type, doors, etc.