Adding array element to JSON column in PostgreSQL
Just use || to concatenate jsonb, if you're using 9.5+.

SELECT lhs::jsonb || rhs::jsonb
FROM ( VALUES 
  (
    '[{"iddata":"1","valor":"18"},{"iddata":"2","valor":"10"},{"iddata":"3","valor":"N/A"},{"iddata":"4","valor":"53"},{"iddata":"5","valor":"Agencia Vespucio Oeste"}]',
    '{"iddata":"6","valor":"ACTIVO"}'
  )
) AS t(lhs,rhs);


Outputs:

[{"valor": "18", "iddata": "1"}, {"valor": "10", "iddata": "2"}, {"valor": "N/A", "iddata": "3"}, {"valor": "53", "iddata": "4"}, {"valor": "Agencia Vespucio Oeste", "iddata": "5"}, {"valor": "ACTIVO", "iddata": "6"}]


Note the explicit cast should not be required. I suggest storing your types as JSONB to start out with, unless you have good reason not to.