Integrating JSP / Tapestry

We have a fairly large site that was built using JSP (before I was here).
However, we are about to change out the "second half" of our pages (order
checkout pages). I was wondering if it is possible (and how painful) to
switch to Tapestry for our last set of pages. We have info in session and
request (order info) that would need to carry through to the Tapestry pages.

Is Tapestry all or nothing? How can we start moving to Tapestry? 

Thanks,
Joel Trunick
http://www.smartprice.com/