It'd be helpful to describe what you're actually trying to accomplish; as Bozho says you can't really share session objects between apps.
You could, however, use JMS (or any other intra-app comms) to send data from one app to another. You'll still need the capability to decide what to do with that data once you have it in the receiving app: how do I associate it with a given user, how do I get it into that user's session, and so on.
User information can be passed in the message, but there has to be some commonality between the two systems, some agreed-upon key, that can be used to figure out who the info belongs to.
Once you have that, the rest is mechanics; there are interesting games to be played, and it's easy to mess it up :)