Tools for the analysis and modeling of complex systems must be able to support the extensibility of formalisms, reusability of models and customization of formalism compositions. From this perspective, SIMTHESys (Structured Infrastructure for Multiformalism modeling and Testing of Heterogeneous formalisms and Extensions for SYStems) is a new approach to the specification of performability oriented formalisms and the evaluation of models. Its originality emerges from the explicit definition of both syntax and evolution semantics of the considered formalism elements. The solution of models is made possible by using a set of non-specialized solving engines used to generate automatically formalism-specific reusable solvers. This paper explains how formalisms can be created in SIMTHESys by showing how three widely known modeling languages are successfully implemented.