How does one generate primary key values for a table?

Create your table with a NOT NULL column (say SEQNO). This column can now be populated with unique values:

SQL> UPDATE table_name SET seqno = ROWNUM;

or use a sequences generator:

SQL> CREATE SEQUENCE sequence_name START WITH 1 INCREMENT BY 1;

SQL> UPDATE table_name SET seqno = sequence_name.NEXTVAL;

Finally, create a unique index on this column.