I thought to create an additional JBoss application server configuration one would have to use some kind of administration tool.

It turned out to be much simpler.

cp -R server/default server/betweengo

If you want to create an ATG application server configuration you can do this.

cp -R server/atg server/betweengo

The only difference between the default server configuration and the atg server configuration is that the latter has two additional datasource XML files for communicating with the SOLID database.

atg/deploy/atg-apps-solid-ds.xml
atg/deploy/atg-solid-ds.xml

For further reading please see JBoss configurations to run an application (need active ATG support contract to see this document) or Building Your Own JBoss Configuration or Using JBoss Application Server.