"Wrapping" a website to display relevant information

I am fairly new to web development and i have recently came across an idea for a project i wish to create.

The project is centered on a specific website which contains Users, Posts and chat per Post. I would like to create an add-on of sorts, that when downloaded, will give the user added functionality to the site.

The added functionality: Supply the user with his own personal "black list" of users. The user wont see the comments posted by blacklisted users. i.e., add an extra button on each comment in the HTML form.

Further more, i would like to keep all the current functionalities in the site, such as posting a comment and updating continuously from the server.

I have asked for permissions from the site managers and am waiting a reply.

Is it possible to create such a thing? If you could point me in the right direction it would be greatly appreciated.