If you want to use a Porche Spyder in the Single Player mode then you have to complete the first era and then buy a used or new Spyder.
Then go on Single Player and it comes up with the cars you've got at the top, pick Spyder and you can now race with the first league Bonus car.
More tricks, tips and cheats for this game are right here - Need for Speed: Porsche Unleashed cheats
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