It seems what happens is you must make use of the org.eclipse.wst.server.discovery plugin. This plugin has a file (discovery.xml) which has "sites", and those sites (such as http://public.dhe.ibm.com/software/websphere/wasce/updates/) seem to be an eclipse update site.

The entry point for this wizard is in the New Runtime wizard. At the top is a hyperlink that reads "Download other adapters" (or something similar). It then queries these sites, and allows you to select one of those available.

It seems the objects are then downloaded as IInstallableUnit's, which is in the org.eclipse.equinox.p2.metadata package.

I kinda lose track of it here, but, I suspect that after this it is downloaded as an actual eclipse bundle and added into the eclipse installation. But I guess I could trace through further

Changes to our site would require adding the category and adding it to certain features in our site.xml.