Alter table Add Primary Key

Hello,

I'd like to know if is possible create a primary key in a table that already
exits in database (note: in Oracle is possible), because I try do that and I
received an ERROR MESSAGES

Example:
/*Script taht I used to create the table*/
CREATE TABLE tespri
(
pri NUMERIC(3,0) NOT NULL CHECK (pri between 0 and 255),
sec VARCHAR(4) NOT NULL CHECK (sec<> 'ana')
);

/* End of Script */

teste=# \d tespri
Table "tespri"
Attribute | Type | Modifier
-----------+----------------------+----------
pri | numeric(3,0) | not null
sec | character varying(4) | not null
Constraints: ((pri >= '0'::"numeric") AND (pri <= '255'::"numeric"))
(sec <> 'ana'::"varchar")

teste=# alter table tespri add constraint ck1 PRIMARY KEY(pri);
ERROR: ALTER TABLE / ADD CONSTRAINT is not implemented for that constraint
type.
teste=#

/* NOTE: database name teste table name tespri */

Thanks for everything.
Best Regards,
Cláudia

Ana Cláudia Dias
Aveiro - Portugal

Email: ana-b-dias(at)ptinovacao(dot)pt