First of all I wish you good luck and not too much overtime. Adding a BPM engine later to an existing project is at least ...interesting.
Nevertheless I used Activiti with JEE (Glassfish 3.x) in some projects (http://www.activiti.org/) with good results. Activiti provides a runtime which can be integrated in your JEE application. It provides a (more high-level) web based Activiti Modeler where business people provide the process. This needs to be technicaly enriched with the Activiti Designer (Eclipse Plugin). You will then deploy the process to the engine.
Initial setup: a few hours. Fully working production environment with real processes: if you do it the first time ~a few month
There is also a quite good hands-on book available from Manning Publications: Activiti in Action