declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE CORTE_TURNO ADD (NUMFECASESOR NUMBER(10))';
    exception when column_exists then null;
end;
/

declare
    column_exists exception;
    pragma exception_init (column_exists , -01430);
begin
    execute immediate 'ALTER TABLE USUARIO ADD (NUMFECHAMENTO NUMBER(10))';
    exception when column_exists then null;
end;
/