Need for Speed - The Run Supercar Pack DLC SP?

I own the normal edition of NFS: The Run on Xbox 360. I'm interested in the Supercar Pack, but here's the problem: Can I use the cars in Single Player (during the "Run")? And if I can, when can I use them? They'd be a big help beating the Extreme-difficulty. If they're not available during the Run, I don't think I'll buy the DLC.