The session transformer in a pipeline manipulates the session.

You give it an input document:

<work xmlns:s="http://apache.org/cocoon/session/1.0">
<s:createcontext name="mystuff"/>
<s:setxml context="mystuff" path="/">
<stuff/>
</s:setxml>
<s:getxml context="mystuff" path="/"/>
</work>

And it does what you say.