I don't know much about profibus but I do know that the Arduino USB is just a lightweight, well in USB terms anyway, USB/Serial converter.
I doubt that will help you much.
This http://www.westermo.net/Resource.phx/content/products/fieldbus/converters/fd-40.htx on the other hand will probably do what you need.
There will be many more but Westermo are generally regarded as being, or at the vary least, working to the de-facto standard for this kind of industrial coms kit.
Of course if what you actually meant was how can I get this stuff to communicate without hocking the farm then that is a whole other ball of wax $)
Al