possibility of adding gtalk contacts to my whatsapp
Nope. It's not possible. A key reason for this is (at least) that 


GTalk identifies people with email address while 
WhatsApp identifies people by their mobile number. 


On top of this both of the systems are proprietary and I don't think neither Google nor WhatsApp would want their users to be pushed over to the other system.

The above said, if you have the mobile number(s) of your GTalk contacts then those will, of course, be added to WhatsApp automatically if they also use WhatsApp. But that's really just stating the obvious.