Multi-agent systems (MAS) involve a wide variety of agents that interact with each other to achieve their goals. Usually each agent has a particular internal architecture defining its main structure that gives support to the interaction among the entities of MAS. Many modelling languages have been proposed in recent years to represent the internal architectures of such agents, for instance the UML profiles. In particular, we highlight MAS-ML, an MAS modelling language that performs a conservative extension of UML while incorporating agent-related concepts to represent proactive agents. However, such languages fail to support the modelling of the heterogeneous architectures that can be used to develop the agents of an MAS. Even worse, little has been done to provide tools to help the systematic design of agents. This paper, therefore, aims to extend the MAS-ML metamodel and evolve its tool to support the modelling of not only proactive agents but also several other architectures described in the literature.