Modeling the domain of a system to be implemented is a very critical and often neglected activity during requirements engineering. In this paper, we set the scene for an approach to complement the SysML/KAOS goal model of a system by adding an ontological representation of its domain knowledge. We think that an Event-B formalization of that domain representation can be used to enrich the formal specifications obtained from the goal model. This paper describes the metamodel that we propose for the representation of domain knowledge and illustrates the proposal through a Landing Gear System case study