is there a way to add thunderbolt interface to existing PCs??
nope. It is useless as of now.
USB 3.0 provides enough bandwidth at the moment and Displays can always use the Display Port in graphic card.
Thunderbolt is basically a fast peripheral port combined with Display Port.
You might see PCI-express add-on cards sporting Thunderbolt soon.
Nope. Not possible. Usb 3.0 is enough bandwidth for most people till now. But yeah we may see it coming in PCI express slots soon.