code: public void singleStepConference(
              String connId,
              String destination,
              KeyValueCollection userData
      ) throws WorkspaceApiException {
          this.singleStepConference(connId, destination, null, userData, null, null);
      }
docs: Perform a single-step conference to the specified destination. This adds the destination to the
  existing call, creating a conference if necessary.
  @param connId The connection ID of the call to conference.
  @param destination The number to add to the call.
  @param userData Key/value data to include with the call. (optional)