Even if model-driven techniques have been enabled the centrality of the models in automated development processes, the majority of the industrial settings does not embrace such a paradigm due to the procedural complexity of managing model life cycle. This paper proposes a semi-automatic approach for the completion of high-level models of critical systems. The proposal suggests a specification guidelines that starts from a partial SysML (Systems Modeling Language) model of a system and on a set of requirements, expressed in the well-known Behaviour-Driven Design paradigm. On the base of such requirements, the approach enables the automatic generation of SysML state machines fragments. Once completed, the approach also enables the modeller to check the results improving the quality of the model and avoiding errors both coming from the mis-interpretation of the tool and from the modeller himself/herself. An example taken from the railway domain shows the approach.