This paper describes an extension to Lustre to support the analysis of globally asynchronous, locally synchronous (GALS) architectures. This extension consists of constructs for directly specifying the timeout automata used to describe asynchronous communication between processes represented by Lustre nodes. It is implemented using an extensible language framework based on attribute grammars that allows such extensions to be modularly defined so that they may be more easily composed with other language extensions. Key words: synchronous languages, extensible languages, attribute grammars, composable language extensions