Commercial simulation environments offer model developers the ability to compose simulation models using generic or domain specific model constructs. Most simulation environments even offer the possibility to compose custom extensions to the simulation environment for faster development of simulation models for a specific domain. This paper evaluates the functionalities for usage and development of custom domain specific extensions that 10 commonly used simulation environments provide to model developers. The findings are scored against a set of criteria, showing that currently more than half of the most used simulation environments offer support to model developers regarding domain specific extensions.