Relying on an IMS/Web 2.0 converged environment, this paper proposes an extensible service composition model which uses the concept of policy for encapsulating the service composition information. Then based on this model, the policy concept is enhanced by being divided into two types (abstract service policy and concrete service policy) for realizing the automatic service composition which mainly relates to the service update and service creation processes. Moreover, an active approach, a passive approach, and a hybrid approach are proposed for balancing the latency and real-time characteristic. With these enhancements, both user's experience and involvement are greatly improved, the service lifecycle is optimized, and the costs for service creation and maintenance are decreased