Hi R.J.,
no, that's not possible. What you could do is make a new asynchronous service that uses the same message format for the request. The way you would go about doing that depends on how your sync service was designed, eg. uploaded from external wsdl, defined in PI, generated from Function Module..
Or in case I misunderstood: if you want a sync service, you have to change the definition similarly.
kr,
Peter