A Struts application is Web application that interacts with humans. A web service interacts with programs. Therefore, turning a Struts application into a web service wouldn't really work.
However, a well-designed Struts app will have a model that has no dependencies on Struts or any other view framework. It would therefore be relatively easy to take some of those model classes and methods and give them a web service interface using one of the many tools available for turnig Java classes into web services.