PHP is a server-side programming language and the $_SESSION superglobal is only directly accessible on the server. With 'normal' php sessions, the data contained in the SESSON superglobal is passed back and forth between the browser and the server in a cookie. So technically, it is possible to modify the session with Javascript in a web browser by modifying the cookie.
But please note, any attempt to do anything like this is probably a terrible idea and there's most likely a far more simple way to accomplish whatever you're trying to do.
Edit: This question I asked may be of use to you
Codeigniter/PHP sessions security question