DECLARE @t TABLE ([Name] varchar(50), Id int);

INSERT INTO @t
VALUES( 'Vasya', 1 ), ( 'Petya', 2 ), ( 'Sara', 3 ), ( 'Chaves', NULL ), ( 'Inga', NULL );

SELECT [Name],
           CASE
                WHEN ID IS NULL 
                THEN ROW_NUMBER() OVER(ORDER BY 1 / 0)
              ELSE ID
           END AS ID
FROM @t;

DECLARE @t TABLE(Name VARCHAR(50), 
                 Id   INT);

INSERT INTO @t
VALUES ('Vasya', 1000), ('Petya', 1400), ('Sara', 1800), ('Chaves', NULL), ('Inga', NULL);
SELECT Name, 
       ID
  FROM @t
 WHERE ID IS NOT NULL
UNION ALL
SELECT Name, 
       ROW_NUMBER() OVER(ORDER BY 1 / 0) + (SELECT MAX(ID) FROM @t)
  FROM @t
 WHERE ID IS NULL;