The complexity of software systems is continuously growing. An important part of this complexity issue concerns the interoperability between existing systems (i.e. legacy systems), where problems often occur due to heterogeneity in e.g. data, involved technologies or models. The Knowledge Discovery Metamodel (KDM) standardised by the Object Management Group (OMG) facilitates representation of existing systems, allowing them to be treated in a homogenous way at the model abstraction level. This paper defines a language suitable for modelling interoperability between these systems by extending KDM and introducing concepts that are specifically aimed at representing relevant interoperability information.