Web Services is an increasingly used instantiation of Service-Oriented Architectures (SOA) that relies on standard Internet protocols to produce services that are highly interoperable. Other types of services, relying on legacy application layer protocols, however, cannot be composed directly. A promising solution is to implement wrappers to translate between the application layer protocols and the WS protocol. Doing so manually, however, requires a high level of expertise, in the relevant application layer protocols, in low-level network and system programming, and in the Web Service paradigm itself. In this paper, we introduce a generative language based approach for constructing wrappers to facilitate the migration of legacy service functionalities to Web Services. To this end, we have designed the Janus domain-specific language, which provides developers with a high-level way to describe the operations that are required to encapsulate legacy service functionalities. We have successfully used Janus to develop a number of wrappers, including wrappers for IMAP and SMTP servers, for a RTSP-compliant media server and for UPnP service discovery. Preliminary experiments show that Janus-based WS wrappers have performance comparable to manually written wrappers.