you can add custom elements to the processdefinition by hand. I if you turn of validation on the server side, you can access these additional elements by retrieving the processdefinition in xml format from the deployed processarchive.
Calculating critical paths is not a feature of jbpm
thank you for your quick answer! I will try the hand job.
Is it planned for a further release to calculate cpm?
no.... it is not planned. Look in the jira (detailed) and the wiki (global) for the roadmap