A very promising module is being developed at the moment - Maestro (http://nextide.ca/solutions/maestro). It serves the purpose of developing a complex workflow on a Drupal platform very well. Unfortunately it's only available for Drupal 7.x, which might be a problem if the goal is to integrate it into a system that runs on an earlier version of Drupal.
As described in this thread: http://drupal.org/node/328145 it is possible to model the business process in a program like MS Visio and use Hyperlinks to connect the different nodes in the model to pages in Drupal.