a straight answer to your question would be, sure. Just create a metadata for dataset (ISO19115) document in geonetwork for your (postgis) dataset using the geonetwork forms. But by reading the replies I noticed you probably wanted to ask: Can Geonetwork harvest the contents of a PostGis dataset (to be used as for example keywords in a metadata document)?
In that case the answer would be no. Geonetwork currently does not offer functionality to harvest metadata from postgis. But hey, what's the use-case here? Where in PostGis is the actual dataset-metadata to harvest (author, organisation, license). Usually one would add this metadata separate to the actual data in for example GeoNetwork...
But in case you just want to retrieve table-structure and contents to be used as keywords with your metadata. Have a look at OGC WFS GetFeature harvesting (GN-trunk/2.8RC1 to be released this weeks) or use a feature-catalog