Any spatially indexed catalog can be viewed as a layer in any WMS-enabled GIS. A spatial index is a special database that allows quick retrieval of images in response to geospatial queries. It tells Express Server everything it needs to know to assemble the images in a catalog into a georeferenced mosaic that can be requested as a WMS layer. Thus, the addition of a WMS layer is simply the addition of a spatially indexed catalog.
See Adding a Catalog and Indexing a Catalog.