afaik there is no global option to configure the session timeout in AS7. If you end up configuring the session timeout inside the web.xml descriptor you might take a look into deployment overlays:
Deployment overlays are are way of 'overlaying' content into an existing deployment, without physically modifying the contents of the deployment archive. Possible use cases include swapping out deployment descriptors,...
Since you have to duplicate and swap out the whole descriptor this is not a very convenient way to override just a single property.