Implementing an extension of a legacy operating system requires knowing what functionalities the extension should provide and how the extension should be integrated with the legacy code. To resolve the first problem, we propose that the use of a component model can make explicit the interface between an extension and legacy code. To resolve the second problem, we propose to augment interface specifications with rewrite rules that integrate support for extensions in the legacy code. We illustrate our approach using extensions that add new scheduling policies to Linux and prefetching to the Squid Web cache. In both cases a small number of rules are sufficient to describe modifications that apply across the implementation of a large legacy system.