We have adopted BPM open source framework Activiti's interface for our implementation of Workflow service. The assumption is that since Newgen's workflow management system is based on same specification as well, we will be able to have a not-too-different process management constructs. However if this turns out to be false, there is significant effort that either partner will have to invest to make it work or we will need to make changes to make it modular enough to be used out of the box.
Have a meeting with Newgen or validate contract early on to ensure that both are compatible. The sooner this exercise can be done, the better chance we will have at reacting to the needed change.