In e-Science, a Grid environment enables data and computing intensive tasks and provides a new supporting infrastructure for scientific experiments. Scientific workflow management systems (SWMS) hide the integration details among Grid resources and allow scientists to prototype an experimental computing system at a high level of abstraction. However, the development of an effective SWMS requires profound knowledge on both application domains and the network programming, and is often time consuming and domain specific. Integrating mature implementations of domain specific SWMS improves reusability of workflow resources and promotes a generic framework for different e-Science domains. In this paper, we discuss different options to derive a generic workflow management system from domain specific implementations, and propose a workflow bus based solution, called VLE-WFBus. Legacy SWMSs are wrapped as federated components and are loosely coupled as one workflow system via a runtime infrastructure. An agent based prototype is presented; the integration among different workflow management systems has been demonstrated.