Create a GeoNetwork OGC Harvest Node (WMS or WFS GetCapabilities to ISO 19139 metadata) through xml.harvesting.add request
GeoNetwork offers a harvest service that, among other, follows a OGC WMS or WFS GetCapabilities URL and transforms the response into an ISO 19139 metadata record. Bear in mind that the resulting ISO metadata record is only as good as the GetCapabilities response and that the metadata can never be entirely ISO 19139 conformant without dummy values due to limitations of the OGC GetCapabilities schema.
GeoNetwork offers a handy user interface to add those "GeoNetwork Harvesting Nodes" and also exposes their functionality through a even handier XML service. See chapter 19.3 on "Harvesting Services" of the GeoNetwork opensource V 2.4 The Complete Manual. Following are my notes on creating an OGC harvesting node through GeoNetwork's harvesting service.