Most existing dialogue systems have been developed specifically for some predefined task domain(s), thus the issues of extensibility and reuse of the dialogue model are often not given much attention. In this paper, we describe our work on extending an existing agent-based dialogue model for e-mail management to further handle the calendar task domain. We discuss how our approach can achieve modularity of the dialogue model at the level of agent plans, facilitating the reuse of discourse-level plans and enabling the extension of the dialogue model to cover different application domains.