When using formal methods, one of the main difficulties is to elaborate the initial formal specification from informal descriptions obtained during the requirements analysis phase. For that purpose, we propose a goal-based approach in which the building of an initial formal model (in Event-B) is driven by a goal-oriented requirements engineering model (SysML/KAOS). In a previous work, we have defined a set of rules to derive a partial Event-B specification from a goal model. In this paper, we propose to enhance the goal model in order to obtain a more complete formal specification. First, we advocate the specification of a domain ontology in order to share common understanding of the structure of the different applications of the underlying domain. This is particularly useful for complex systems to explicit and make clearer the domain knowledge. For a specific system, a class and an object diagrams are then specified to detail its components and their relationships. Finally, we describe how the ontology and the structural model are translated into Event-B. The proposed approach is illustrated through a landing gear system