Change existing column in PG to auto-incremental primary key
Create a sequence and use it as the default value for the column:

create sequence measures_measure_id_seq
   owned by measures.measure_id;

alter table measures
   alter column measure_id set default nextval('measures_measure_id_seq');

commit;


That essentially what serial does. 

See the manual for details:
http://www.postgresql.org/docs/current/static/datatype-numeric.html#DATATYPE-SERIAL