You can try to use VH Multi Camera Studio for converting RTSP stream into a virtual webcam video supported by many programs. It works fine with Skype.

How to use RTSP webcam in programs not supporting IP web-cams:

1. Download and install VH Multi Camera Studio
2. Launch VH Multi Camera Studio.
3. Click "Add URL" and in the "Select URL to capture from" editbox enter your rtsp:// URL.
4. Hit the "Ok" button. You should be able to see your web-cam video now.
5. Check the "Enable Output to VHMulticam Video Capture Device" and the "Default source (VHMulticam connects to this)" checkboxes.
6. In Manycam or any other program (including Skype) use "VHMultiCam" as a web-cam device.

Note that VH Multi Camera Studio doesn't seem to support h.264 rtsp:// videos from web-cams.