That’s correct. When you buy a Mac, you don’t need to download extra drivers. Only if you buy a Mac Pro and later install a different (or additional) PCI graphics card, you may need to download drivers.
The only PCIe expansion capability available on any mac (including the new mac pro) is thunderbolt. The new mac pro does not have any internal expansion.
You can however, use something like [this](http://www.sonnettech.com/product/echoexpresschassis.html), as long as you realize that you only have access to PCIe x4 bandwidth.
I've also not seen any posts of anyone actually doing this yet.