Run the following command as dba:
set option public.default_dbspace = '<your NLS dbspace>';
and re-connect the NLS user.
You can also set it at the user instead of public level, but this ensures that any connect user issuing a DDL command will operate in the other db_space.
Note: objects already in IQ_SYSTEM_MAIN will stay there - including existing tables that NLS might add partitions to. I think you have to recreate (under a new name), copy the data, and use sp_iqrename on the new table to rename it to the old name, after you drop the old table - to move the table to the new dbspace.
Chris