Actually...
You can use the cars, you just have to replace every part code in a car (There are dozens and dozens of part codes for Every. Single. Car.) with legitimate part codes you have to look up online or find yourself (good luck with that), give the car legitimate codes to make the game recognize name/body/manufacturer/etc., enter ALL settings in a workable manner (including the mysterious settings that are not viewable or adjustable in-game and which no one knows what they do) along with other miscellaneous bits of information the game looks at when you're using a car, making sure you don't make any typos or location mistakes at all during the two or three hours of work you just did, just to make a single car useable. Again, good luck with that. It's a real pain to do and completely not worth it, especially when you can just buy an already assembled car and make whatever changes you want. Also, the extra garage spaces are probably overwriting something else, so there's that too. It's not even really worth experimenting with, and I don't know how stable the cars will be after the modifications you do, either. I experimented with it once (got the code from either Skywwalker's site or James1058's site, I don't remember), and that was more than enough.
In short...
It's good for nothing but bragging rights, and that's not particularly impressive with a game over a decade old. Want more cars you can actually use without going through a ton of completely unnecessary trouble? Get more memory cards.
(sorry for the formatting-whore, my Science teacher often says like the bolded, underlined, and italicized alot)
