The answer is actually very simple, and is based on a misconception you seem to be having:
Can Virgin Galactic's Orbital ship reach ISS
SpaceShip2 is not an orbital vehicle. The ISS is in orbit, SS2 cannot go to orbit, therefore, SS2 cannot go to the ISS.
In fact, you answer the question yourself:
I have had read that the SpaceShip2 will have merely 6 mins to spend in Space i.e It will go upto 100km for few minutes and then plunge back into the atmosphere!
The ISS is in orbit at 320km – 430km, so SS2 is about 200% to 300% short.
But more importantly, the ISS is in orbit, which means it is going sideways at 27000km/h, whereas SS2 is going sideways at … 0km/h. It is just going up and then falling down.
Obligatory Randall Munroe shoutout: https://what-if.xkcd.com/58/
Space is not about going high. Space is about going sideways really fast, so that you are constantly falling towards Earth but you are moving so fast sideways that you are always "missing". It is not like as soon as you get to the ISS, gravity just turns off. Gravity on the ISS is roughly 90% the gravity at sea level.
If you are in the central US, the ISS is closer than the ocean. Since you can get to the ocean in your car, distance is clearly not the problem.
There is another misconception in your question:
It will go upto 100km for few minutes
That sounds like SS2 is going to up to 100km, then cruise around at that altitude for a bit, and then go back down. That is not how it works.
SS2 will mostly be on a ballistic trajectory. It's really not much different from throwing a ball up in the air. The ball will rise but lose velocity, then it will reach its highest point, stand still for an infinitely short moment, and then accelerate back down.
The same is going to happen with SS2. It will go straight up (unlike the ball, it will have a rocket motor that accelerates it, whereas the ball's acceleration stops as soon as it leaves your hand, but as soon as the rocket burns out, there's no difference anymore between the SpaceShip2 and the ball), and then fall back down again.