Simulation can be considered as a tightly coupled and iterative three-staged process composed of model design, model execution and execution analysis. In the model design phase, conceptual simulation models have been extensively used as the cornerstone specification formalism. All static and dynamic knowledge about the real system must be encoded in some form, to allow specification of interaction without necessarily specifying the dynamics in quantitative terms. HOOMA (Hierarchical Object-Oriented Modelling Approach) was initially proposed as a conceptual model to specify large-scale simulation models based on queuing networks. In this paper we present a domain extension to the Unified Modelling Language (UML), termed DExOM/UML, as a formalism orthogonal to HOOMA but that jointly permits to represent both the architectural and interaction dimensions of a queuing network simulation model.