Hi Fabrizio!
WebcamXP do not have support for the ONVIF standard. Netcam Studio have that.
If the ip cam do not have a template in webcamxp you can use the Moonware Universal Source Filter found here webcamXP - Download
Download and install and in webcamxxp you can connect the IP cam using the URL to the cam like rtsp;//xxxx
Here is a guide how to use the filter
Moonware Universal Source Filter (Directshow Filter)
-Henrik