As you can see in Externalizing Internal Database Connections and Internalizing External (Shared) Database Connections), in both cases, the context menu contains two interesting items: the and items. These give you an opportunity to browse a database (if your connection is valid) and/or extract metadata from some selected database table. Such metadata will be internal only, but you can later externalize and/or export them.
Remember that you can also create a database table directly from metadata. See Create Database Table from Metadata.