Hi Yoann,
I have done something similar with a home grown REST service. In your URS Web Service block, just choose HTTP instead of SOAP. I didn't bother trying to pass JSON in the message body of the request, instead I wrapped everything as query string parameters. If your web service responds in JSON format, you can likely assign the output value to a variable and write a subroutine that will parse the JSON for you into KVPs. I agree though it would be cool to have this a little more natively. You should open a feature request.