Quite simply no.
Javascript, a engine which runs in the client browser sandbox, does not have access to the server hosted session object. Nor should it. Just think about the security implications?
My advice would be to use AJAX. Write JS to make an AJAX call to purposely designed page for modifying the session.
But I'd still be concerned about this too as if I found it whilst interrogating your page using my browsers developer tools, I could quite easily abuse it and make modifications to my session on your server.
Depending on what your site does, say a shop? I could hack it and order myself lots of free products!
When thinking about website security you should take a look at OWASP and PCIDSS.
https://www.owasp.org[
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https://www.pcisecuritystandards.org/security_standards/index.php[
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