If you have the column and the sequence, you first need to populate a new key for all the existing rows. Assuming you don't care which key is assigned to which row
UPDATE table_name
SET new_pk_column = sequence_name.nextval;
Once that's done, you can create the primary key constraint (this assumes that either there is no existing primary key constraint or that you have already dropped the existing primary key constraint)
ALTER TABLE table_name
ADD CONSTRAINT pk_table_name PRIMARY KEY( new_pk_column )
If you want to generate the key automatically, you'd need to add a trigger
CREATE TRIGGER trigger_name
BEFORE INSERT ON table_name
FOR EACH ROW
BEGIN
:new.new_pk_column := sequence_name.nextval;
END;
If you are on an older version of Oracle, the syntax is a bit more cumbersome
CREATE TRIGGER trigger_name
BEFORE INSERT ON table_name
FOR EACH ROW
BEGIN
SELECT sequence_name.nextval
INTO :new.new_pk_column
FROM dual;
END;