Sessions are configurable in eZ Publish 5.2 by defining the session_name setting under your siteaccess group configuration in your config/ezpublish.yml file.
Here's a usage example:

ezpublish:
    siteaccess:
        list:
            - ezwebin_site
            - eng
            - ezwebin_site_admin
        groups:
            ezwebin_site_front_group:
                - ezwebin_site
                - eng
        match:
            URIElement: '1'
    system:
        ezwebin_site_front_group:
            session_name: <YOUR_SESSION_NAME>
        ezwebin_site_clean_group:
            database:
                type: mysql

 For better integration between 5.2 (symfony based) kernel and legacy (4.x) kernel, injection is used to inject settings, session and current siteaccess from 5.2 into legacy using an event: kernel.event_subscriber

Injected session

When eZ Publish legacy (4.x kernel and extensions) are running via the new eZ Publish 5.2 kernel, then the Symfony session is injected into it and all session related settings in legacy is ignored.

For more details on how to configure sessions in your eZ Publish 5.2 setup, please refer to the documentation of Symfony HTTP Fundation component.