Hi Vivek,
PI-WebParts are developed by OSIsoft thus you would not be able to extend them, you could file enhancement requests with OSIsoft. Note, the next versions of PI-WebParts will be based on Silverlight technology.
An option for you would be to create you own web parts from the ground up that interact with PI-WebParts. In this manner you would connect to a PI-WebPart, get the relevant data and then your own developed web part does what it needs to with the data.
Hope this helps.
Principal Consultant
Real-Time Data Management @ Wipro Technologies