Modeling in real industrial projects implies dealing with different models , metamodels and supporting tools. They continuously have to be adapted to changing requirements, involving (often costly) problems in terms of traceability, coherence or interoperability. To this intent, solutions ensuring a better adaptability and flexibility of modeling tools are needed. As metamodels are cornerstones in such tools, metamodel extension capabilities are fundamental. However , current modeling frameworks are not flexible or dynamic enough. Thus, following the ongoing OMG MOF Extension Facility (MEF) RFP, this paper proposes a generic lightweight metamodel extension mechanism developed as part of the MoNoGe collaborative project. A base list of metamodel extension operators as well as a DSL for easily using them are introduced. Two different implementations of this extension mechanism (including a model-level support when (un)applying metamodel extensions) are also described, respectively based on Eclipse/EMF and the Modelio modeling environment.