DECLARE @t AS TABLE
                    ([товар]  CHAR(1) NOT NULL
                   , [неделя] TINYINT NOT NULL
                   , [колво]  INT NOT NULL
                   , PRIMARY KEY CLUSTERED([товар], [неделя])
                    );

INSERT INTO @t(
       [товар]
     , [неделя]
     , [колво]
              )
SELECT 'a',  1,  10 UNION ALL 
SELECT 'a',  2,  15 UNION ALL 
SELECT 'a',  4,  20 UNION ALL 
SELECT 'a',  5,  10 UNION ALL 
SELECT 'b',  1,  5  UNION ALL 
SELECT 'b',  4,  25 UNION ALL 
SELECT 'b',  5,  30;

WITH weeks
     AS (
     SELECT 
            [товар]
          , 1 AS             [неделя]
          , MAX([неделя]) AS aw
     FROM 
          @t
     GROUP BY 
              [товар]
     UNION ALL
     SELECT 
            [товар]
          , [неделя] + 1
          , aw
     FROM 
          weeks
     WHERE [неделя] < aw)
     SELECT 
            ISNULL(t.[товар], w.[товар]) AS   [товар]
          , ISNULL(t.[неделя], w.[неделя]) AS [неделя]
          , ISNULL(t.[колво], 0) AS           [колво]
     FROM 
          @t AS t
          FULL JOIN weeks AS w ON w.[товар] = t.[товар]
                                  AND w.[неделя] = t.[неделя]
            ORDER BY 
                     [товар]
                   , [неделя] OPTION(
                                     MAXRECURSION 0);