You can use the MATERIALIZE_DEFERRED_SEGMENTS procedure in the DBMS_SPACE_ADMIN package to create segments for tables and dependent objects for tables with the deferred segment property.
You can also force the creation of segments for an existing created table and table fragment with the DBMS_SPACE_ADMIN.MATERIALIZE_DEFERRED_SEGMENTS procedure. The MATERIALIZE_DEFERRED_SEGMENTS procedure differs from the ALTER TABLE MODIFY PARTITION ALLOCATE EXTENT SQL statement because it does not allocate one additional extent for the table or table fragment.
Oracle Database PL/SQL Packages and Types Reference for more information about the DBMS_SPACE_ADMIN package