Model Driven Engineering aims at changing the focus from code to models. To achieve it, enabling model transformation is essential. A type of transformation is meta-model extension. It is particularly salient for the use of models in defining Domain Specific Modeling Languages, especially for profiling existing languages. Meta-models describing language syntax have a low number of components. Accordingly, an expert-driven approach to extending meta-models is both practicable and preferable to an automatic one, which has a higher level of inaccuracy. We propose in this paper three principles for aiding an expert in practically extending meta-models with domain specific concepts. The resulted language profiles are backwards-compatible. We apply these principles to defining an ArchiMate profile for telecommunications service creation.