I have an existing eclipse with a bunch of plugin installed,so I want to know:
Is it possible to install cuda nsight in an existing eclipse installation?
If so, how?
No, that isn't possible.
NSight Eclipse Edition is a heavily customised fork of Eclipse. You cannot integrate it into an existing standard Eclipse installation. You will have to install it alongside any other Eclipse version you have and use it separately.
[Answer assembled from comments and added as a community wiki entry to get question off the unanswered queue for the CUDA tag]