Is it possible to create an extension to be injected as an add-on to a website ?

[manguima]

I need the extension to modify a website and become a complement to it, such as adding new buttons to the menu or even creating a menu on a third-party website

[laurislopata]

+1

[KajKandler]

You can do that with content_scripts. Create the default popup template contains such example code.