The Documentum platform from EMC is a CMS (or ECM) tool used for content management. It uses an external database for storing metadata, but is not a database itself. Documentum will store and manage the content for you and keep the links to the content store (and a bunch of other business policy information) in the database. So to your question, if you wish to include external database tabel(s) inside Documentum tablespace for running DQL queries to use both Documentum object tables and the external table, you need to "register" them so that they become accessible to Documentum's API and DQL. The register DQL syntax is:
REGISTER TABLE [owner_name.]table_name
(column_def {,column_def})
[[WITH] KEY (column_list)]
[SYNONYM [FOR] 'table_identification']
Note, the table should be at least selectable to the docbase owner - the credential of the database user that Documentum uses to connect to it's database.