While there is no single "exclusive" official story mode unlocker, the Test Drive Unlimited 2 (TDU2) Save Editor
Within the TDU2 community (still active on Discord and Reddit r/tdu2), the save editor is widely accepted because: tdu2 save editor pc exclusive
Conclusion
Crucial: Copy your savegame.bin to your desktop. If the editor crashes, you can restore this backup. While there is no single "exclusive" official story